The restless nature of AGN: 10 years later

Determining the inner geometry of AGN - i.e. the size and location of the central X-ray emitting corona relative to the accretion disc, the shape, size and structure of the disc, the location of the broad line region and its possible connection with disc winds, the location and structure of obscuring material - remains one of the main challenges of astrophysical research. Apart from M87 and SgrA*, whose very inner regions have been imaged by millimetre global VLBI, these inner structures are far too small for direct X-ray or optical imaging. We therefore use the technique of 'reverberation mapping'. Here the time lag between direct X-ray emission and lower energy (UV/optical) emission, produced by reprocessing of X-rays by the surrounding material, gives us the distance from the central X-ray source to the surrounding material. By measuring that lag in multiple wavebands, corresponding to material at a range of temperatures, we are able to map out at least the temperature structure of the surround material. We can then compare our observed structure with the structure that we expect based on theoretical models of these structures and hence determine whether the models are correct. I will review the observations that have been carried out and their implications for our understanding of the inner geometries of AGN.

AGN variability carries information about the geometry of the accretion flow which is usually unaccessible to direct imaging methods. In particular, the reverberation signals at optical wavelengths of reprocessed high-energy photons provide insight into the size of the disc itself as well as inclination, mass accretion rate and the temperature profile of the disc itself. Over the last decade, we have developed the intensive broadband reverberation mapping technique combining the capabilities of Swift and Las Cumbres Observatory (LCO) to perform long-term (multiple years), high-cadence (sub-day), multi-wavelength (X-rays to NIR) experiments on ~30 local AGN. These IBRM experiments have enabled stringent tests on the predictions of accretion theory and shown disagreements, catapulting new avenues to understand AGN variability. In this talk, I will review the main outcomes of the past IBRM campaigns, with particular focus on a high Eddington accretion source PG 1119+120. Our spectral and temporal decomposition allowed us to retrieve the signal of two components; a fast signal consistent with the X-ray reprocessing scenario, and a slow signal with a spectral energy distribution consistent with diffuse continuum emission from the broad-line region. I will also present the launch of our open-source database - AGN Variability Archive (AVA). This legacy database of processed light curves and spectra of ~10 years of observations taken by both Swift and LCO will enable further studies in accretion flows of supermassive black holes.

Over the last decade reverberation mapping (RM) campaigns of active galactic nuclei (AGN) have enabled us to probe their inner regions in unprecedented detail. Whilst observations have broadly confirmed that the short-term variability of the accretion disc is driven by variations in the X-ray corona a number of puzzles have also emerged, including: the contribution of the broad line region (BLR) to measured lags; the implied large disc sizes; the role of disc winds as obscurers or additional reprocessors; and unexpectedly long X-ray to UV lags.

To address these issues I present results from a superb new multi-wavelength data set on the bright Seyfert galaxy MCG$+$08$-$11$-$11. This major, high-cadence monitoring campaign, conducted with Swift and ground-based observatories, captured the source in an unusually highly-variable phase compared with previous observations: rapid, large-amplitude flux changes are observed at all wavelengths. We find that the X-ray and UV-optical lightcurves are much more highly-correlated than typically found in similar RM studies. The wavelength-dependent lags form a spectrum that approximates disc reprocessing predictions. The behaviour of the source was markedly different during an optical RM campaign conducted just a year prior in which only slow and moderate flux changes were seen; the resultant lag spectrum was very much steeper during this period, likely because of a stronger contribution from the BLR. Our new results further emphasise that a simple, static reprocessing geometry cannot explain the observed variability: even in the same source, different reverberating components (or processes) dominate at different times. This rich data set provides a golden opportunity to grapple with the dynamic and complex nature of AGN variability, and I discuss the broader implications.

Quasars are a class of objects in the Universe with very apparent flux variation. UV/optical variability of such sources has attracted particular attention. The radiation in this band is thought to come from the accretion disk. As the central dynamical region of a quasar, the accretion disk is theoretically believed to be related to structures such as the corona and emission line region. Studying the correlations between UV/optical variability and physical processes occurring in these structures can help to constrain and improve the relevant physical models.

We make a one-parameter characterization of UV/optical variability amplitude of quasars using the famous Sloan Digital Sky Survey 10-year light curves in the Stripe 82 region, and explores the statistical correlations between AGN UV/optical variability (from the accretion disk) and X-ray emission (from corona), and between the variability and UV/optical line emission (from line emitting regions). We find that there is an intrinsic positive correlation between UV/optical variability and X-ray loudness, and this correlation occurs mainly at long timescales. This result prefers the physical picture depicted by the thermal fluctuation model of accretion disk, i.e., both the corona heating and UV/optical variability are related to magnetic turbulence in the accretion disk. Then we find a positive intrinsic correlation between the variability amplitude and the equivalent width for the broad Mg II line, the C IV line and the [O III] 5007 line. We point out that the possible physical reason behind such correlation could be: a more variable accretion disk will have a harder and bluer spectral energy distribution and therefore increases the emission line equivalent width; on the other hand, a more variable accretion disk may launch more clouds, thus increasing the covering factor of emission line region.

In the future, with the rapid progress of time-domain surveys, more extensive/systematical studies of AGN variability could significantly promote our understanding of AGN variability and relevant physical processes.

Reverberation mapping (RM) is a powerful tool to explore the unresolved central region of active galactic nucleus (AGN), e.g., accretion disk. Determining the structure of accretion disks in AGN is fundamental to understanding the growth of supermassive black holes, confirming the standard thin disk theory, and examining the X-ray reprocessing variability model. However, recent continuum RM suggests that the observed accretion disk size is around three times larger than prediction. In this talk, I will introduce our recent continuum RM results of bright AGNs in ZTF and the well-known dwarf galaxy NGC 4395. We found that the continuum lag is dominated by the diffuse continuum emission, which may account for the disk-size discrepancy. In addition, we will introduce a new method to measure the reverberation black hole mass via continuum RM.

I will discuss the use of longer timescale “negative” lags, where the variability in high frequency bands lags the corresponding variability at low frequency, as a probe of accretion disk structure. Traditional reverberation mapping uses lags of variations in AGN photometry from high frequency to low frequency wavebands on the light-crossing timescale which come from the reprocessing of light in different temperature regions of the disk. The long negative lag, on the other hand, is due to fluctuations in the outer part of the UV/optical region of the disk that are accreted inward on the inflow timescale. Because the inflow rate also depends on disk properties, unlike the speed of light, these long lags can provide additional information about disk structure. Standard disk models predict the inflow timescale is on the order of hundreds of years. However, recent 3D radiation magnetohydrodynamic simulations of AGN disks and analysis of high-cadence, long baseline observations of Fairall 9 suggest that in the UV/optical region of the disk, the inflow timescale can be on the order of only 100 days, not years. This much shorter lag timescale would make the detection of long lags possible with long baseline observations from instruments such as SWIFT or Vera C. Rubin Observatory. I will outline the underlying theory of these long lags, show results from 3D radiation magnetohydrodynamic simulations of disk models beyond the standard disk model, and also present some candidate long lags.

Recent multi-wavelength surveys of a few AGN have given us the opportunity to constrain well the cross-correlation between the X-rays and the UV/optical variations in these objects. The variations in the UV lead the variations detected at longer wavelengths in almost all cases where good quality light curves, in many wavebands, exist. However, there have been indications that the optical variations lead the UV variations on the longest sampled time scales in a few objects. This could indicate the presence of accretion rate variations which propagate inwards. We use well sampled, long light curves of a few Seyfert galaxies to compute the time lags on both long and short time scales. We will present the results regarding the dependence of the observed time-lags on the probed time-scale, and we will discuss possible constraints on various models for the observed optical/UV variations in AGN.

The variability of Active Galactic Nuclei (AGNs) has been studied for decades, with the UV/optical continuum observed to stochastically fluctuate at the 10% level over timescales of weeks to months. Fundamentally, this variability should be driven by temperature fluctuations in the accretion disc surrounding the central black hole. Where multiband lightcurves are available, the variability is similar in all bands, but the bluer wavelengths vary earlier than the redder wavelengths with delays typical of the light travel time across the disc. These observations have led to the commonly used "lamppost" model, where central luminosity fluctuations - typically linked with X-ray emission - irradiate the disc to drive the UV/optical variability. However, it seems unlikely that this is the only source of variability in the disc. We introduce a new approach to understanding disc variability where we invert the multiband UV/optical lightcurves of AGNs into “maps” of the disc resolved in time and in radius under the assumption of axisymmetry. In addition to a lamppost "signal", we see strong evidence for small amplitude, slow-moving temperature fluctuations. We suggest that these fluctuations dominate AGN variability on long timescales, a hypothesis that will be tested by Rubin/LSST in the near future. This new method for understanding disc variability can also be used to probe other issues, such as the degree to which unrecognized emission from the broad line region (BLR) contaminates continuum lightcurves.

From optical to X-ray, the variable continuum emissions of AGN are generally found to be correlated with variations at longer wavelengths lagging the shorter ones. Both the correlation and the lag-wavelength relation are usually understood within the widespread X-ray reprocessing scenario. However, both of them do not always preserve and challenge the reprocessing scenario. In recent years, we (Cai et al. 2016, 2018, 2020) upgrade the inhomogeneous thermal fluctuation model proposed by Dexter & Agol (2011), by introducing a common larger-scale fluctuation (as a result of the propagation and mixing of local fluctuations, likely, by magnetic fields allover the accretion disk) and suggesting a new origin for the continuum lag (as a result of the differential regression capability of local fluctuations responding to the large-scale fluctuation). Now, the new thermal fluctuation scenario can account for several observational properties of AGN variability, including the correlation and lag across the X-ray/UV/optical and the timescale-dependent color variation, and may shed new light on comprehending the UV/optical continuum variations and the relation to X-ray for AGN.

Several active galactic nuclei (AGN) show UV/optical variability lagging behind the X-ray emission by a few days. The simplest and most straightforward interpretation is that the variable X-ray flux from the corona illuminates the accretion disc below where it is partially reflected and observed as fast X-ray reverberation signal, and partially absorbed and thermalised in the disc, which produces a slow UV and optical reverberation signal. Since the corona is thought to be centrally located and very small compared to the accretion disc, it first illuminates the hottest inner parts of the accretion disc and later on its colder further out areas. Thus one expects to see the original X-ray fluctuations to be firstly followed by variations in the UV and then in the optical wavebands.

In this talk, I will present our newly developed full GR-ray-tracing code that computes the thermally reverberated UV/optical continuum responding to X-ray illumination by a compact corona. Our code considers the mutual interaction of the accretion disc and the X–ray corona. I will discuss how the properties of the system (e.g., black hole spin, accretion rate, corona height, etc.) affect the UV/optical time lags. I will also present our results from modelling the observed lags obtained from long monitoring of bright local AGN.

We measured the dust torus  size of 86 quasars with bolometric AGN luminosity in the range 10^43.4 to 10^46.4 erg/s  by determining the lag between the optical continuum emission obtained from ground-based optical surveys, i.e., CRTS, ASAS-SN, PTF and ZTF, and the mid-infrared continuum observed with the W1 and W2 bands from the Wide-field Infrared Survey Explorer (WISE) survey. By combining the new measurements with our re-analyzed measurements of the sample in the literature, we constrain the torus size - AGN luminosity relation over a large dynamic range of luminosities (i.e., 10^43.4 to 10^47.6 erg/s) with a slope of 0.31 and 0.32, depending on mid-infrared band W1 and W2-band, respectively. We corrected the accretion disk contamination in the observed MIR light flux, obtaining a slightly changed slope of 0.37 and 0.31 for W1 and W2-band lags, respectively. While the new slope is shallower than the value of 0.5 expected from thermal equilibrium model, it is in good agreement with that obtained from the interferometric observations available in the literature. We also found wavelength dependent lags (from K to W1, W2), suggesting a stratified structure of the dust torus, such that emissions in different infrared wavelengths come from the different regions of the torus.

NGC 6814 is a nearby ($z = 0.005$) Seyfert 1.5 galaxy that we recently showed had undergone a rapid X-ray occultation event during an XMM-Newton observation from 2016. The X-ray eclipse of high column ($N_{\mathrm{H}} \approx 10^{23}~\mathrm{cm}^{-2}$), mildly ionised ($\log\xi \approx 1~\mathrm{erg~cm~s^{-1}}$) matter lasted ~45ks, with ingress and egress each lasting ~14ks, revealing a partially covered X-ray region we estimated to be ~25 gravitational radii across. From August to November 2022 we observed NGC 6814 3-4 times per day with Swift to search for new X-ray eclipses to better understand the environment in this AGN. We present here a new analysis of the 2016 XMM-Newton data using X-ray colour-colour diagrams that reveal an inhomogeneous, clumpy obscurer, which is likely embedded within an extended, large scale structure based on simultaneous and long-term Swift coverage. Our 2022 Swift campaign reveals no new X-ray eclipses, but offers a rich data set with which we conduct the first thermal reverberation analysis of this AGN. We find highly correlated optical/UV to X-ray variability that exhibits a significantly flatter time-lag spectrum than the predicted 4/3 power law relation of a standard X-ray illuminated accretion disc. Furthermore, we find that during the 2016 X-ray eclipse X-rays de-correlate from optical/UV variation before resuming highly correlated broad band variability ~30 days later.

High frequency radio emission may originate from scales as small as the
innermost accretion disk, and can thus probe directly the relativistic electrons
and the magnetic fields in the coronal gas of radio quiet AGN.

I will present simulations of the time evolution of the distribution functions of
relativistic electrons following their injection due to a coronal reconnection event.
The electrons cool through Compton scattering, producing a pulse of X-ray
emission, and through synchrotron emission, producing a pulse of high frequency
radio emission. Future simultaneous monitoring of X-ray and mm emission may allow
to probe directly the coronal heating and cooling mechanisms.

I wil also briefly point out the false detections of correlated variability when two
red light curves are correlated, as we found in a recent study of simultaneous radio
and X-ray observation of three AGN. I will also describe how these biases can be
minimised in future studies.

Supermassive black holes preserve information on the growth of the host galaxy and its dynamic evolution. Thus, constraining their parameters is crucial to shed light on their formation and evolution. In recent years, X-ray astronomy has undergone a renaissance, with several instruments that perform large observational campaigns and cover an extremely wide range of energy timescales to study Active Galactic Nuclei (AGN). The X-ray radiation produced by the closest accreting matter to the black hole shows distortions due to the strong relativistic effects. Proper modeling of these features constrains the system geometry and the interplay between the corona and the accretion disk. I will review the recent results of X-ray spectral timing analyses with an emphasis on the interpretation of the observations.

X-ray reverberation mapping studies of AGN can, in principle, be used to measure the black hole mass and spin, the accretion disc and corona geometries, and the ionisation state of the disc. We report on our efforts to fit the spectra and time lags of a number of AGN, but focus primarily on two sources, 1H 0707-495 and IRAS 13224-3809. We can explain the low- and high-frequency lags, find that an extended corona is required, estimate the black hole masses, and find there are different correlations between parameters in each object as their coronae change. However, this model is slow to evaluate, the parameter space had to be limited (e.g., fixed spin; simplified geometry), it was difficult to characterise the error bars and degeneracies in the model, and the data were fairly noisy. We will discuss how new approaches to modelling and improved data will lead to a better understanding of the inner workings of AGN.

The hard X-ray emission universally found in AGN is believed to be produced in the so-called corona, of which the physical nature remains unclear. A fundamental parameter is the coronal temperature ($T_{\rm c}$), which could be measured by fitting the high-energy cutoff ($E_{\rm cut}$) in the hard X-ray spectra. With multiple NuSTAR observations, we search for the variation of $T_{\rm c}$/$E_{\rm cut}$ in individual sources. We get a small sample of several sources, which demonstrate an interesting non-monotonic variation pattern, with a break point of the photon index $\Gamma$ detected. Sources are found to be “hotter-softer-when-brighter” at $\Gamma < 2.05$, but turn into “cooler-softer-when-brighter” at $\Gamma > 2.05$. Such a behavior indicates that multiple mechanisms, for instance, changes of the coronal geometry and the cooling efficiency, are contributing to the X-ray variability in AGN. Meanwhile, we are also interested in how $T_{\rm c}$/$E_{\rm cut}$ differs from one source to another. We measure the $T_{\rm c}$/$E_{\rm cut}$ in a large sample and investigate the correlations between $T_{\rm c}$ and other parameters (photon index $\Gamma$ and Eddington ratio). A strong positive correlation between $T_{\rm c}$ and Γ is detected, while none between $T_{\rm c}$ and Eddington ratio. In other words, counter-intuitively, hotter coronae tend to produce softer spectra, while the accretion rate is not a primary determinant of the coronal temperature.

We present a study of the ensemble X-ray variability of Active Galactic Nuclei (AGN) over a large range of timescales (20 ks ≤ T ≤ 14 yrs), redshift (0 ≤ z ≲ 3), luminosities ($10^{40} \leq L_X \leq 10^{46}\ \mbox{erg s}^{−1}$) and black hole (BH) masses $10^6≤M_⊙≤10^9$). Through the use of the "variance–frequency diagram", as a viable alternative to the power spectral density (PSD), we show that the data collected from archival observations and previous literature studies are consistent with a universal PSD form which does not show evidence for systematic evolution of shape or amplitude with redshift or luminosity. We find new evidence that the PSD bend
frequency depends on BH mass and, possibly, on accretion rate. We will discuss the implications for current and future AGN population and cosmological studies

One of the most influential relations in extragalactic astrophysics is the one that links the stellar-mass component of galaxies (Mstar) to the masses of the supermassive black holes (MBH) at their centres. Observational constraints on the shape, normalisation and redshift evolution of the Mstar-MBH relation provide important clues on the co-evolution of galaxies and their supermassive black-holes. Unfortunately, measuring the Mstar-MBH relation, particularly at higher redshifts, is challenging and prone to systemics. In this contribution I will present a new method that provides a handle on the Mstar-MBH relation by modelling the ensemble variability of X-ray selected AGN samples. A key ingredient of the method is the modelling strategy that links, for the first time, the demographics of AGN to the physics of the stochastic flux variations of accretion flows and allows the interpretation of the variability properties of AGN populations. I will demonstrate the predictive power of the model by comparing in a forward manner with observational measurements of the ensemble excess variance of X-ray AGN in the Chandra Deep Field South. I will also discuss future prospects for joint constraints on both models of AGN variability and the Mstar-MBH relation as a function of redshift.

I will present the analysis of the X-ray variability properties of the Seyfert 1 Galaxies belonging to the BAT AGN Spectroscopic Survey (BASS) using XMM-Newton observations. This sample includes more than 500 observations of 151 local AGN (medium redshift z=0.06). The aim of this work is to constrain the relation between the common estimators of the variability amplitude (i.e., fractional variability and normalised excess variance), calculated in different energy bands, with the physical and accretion properties of AGN such as the black hole masses of the central supermassive black hole (known for all the sources of the BASS sample from either broad Balmer lines or reverberation mapping estimations) and Eddington ratios (estimated combining the black hole masses measurements with the estimates of the bolometric luminosity derived from the BAT 14-150 keV luminosity). As expected from previous studies we find a strong anti-correlation between the excess variance and the black hole mass. We do not find correlation between the excess variance and the Eddington ratio but we find a strong anti-correlation with the 2-10 keV luminosity, which disappears when we removed the dependence of the excess variance on the black hole mass. Exploring the relation of excess variance in different energy bands we found that the variability of the sources of our sample is mostly due to the flux variation of the primary continuum and/or of the reflection component, at least on scale of 10ks. I will also show the comparison of the variability property of the unobscured AGN of the BASS sample with the X-ray variability properties of a sample of 5 Super and Hyper-Eddington sources (1$<\lambda_{Edd}<$426) belonging to the Super-Eddington Accreting Massive Black Holes sample.

Temporal variability of flux across the electromagnetic spectrum is a commonly observed phenomenon in Active Galactic Nuclei (AGN), however the phenomenon is not well studied in low accretion rate AGN, primarily due to difficulties identifying them in X-ray catalogues due to their lower luminosity.
In this work, we use our algorithm EXODUS, which searches for variability in the whole of XMM-Newton's EPIC field of view and is agnostic of source detection and the number of counts. It accomplishes this by binning the observations into short time windows and comparing the pixel counts per window to the median pixel counts to detect variable sources within the observation, making EXODUS ideal for studying faint rapid X-ray transients. We apply EXODUS to all of the observations that comprise the 4XMM-DR11 catalogue to create a reliable subset of low Eddington ratio AGN. Understanding these AGN helps us to develop a more complete framework constraining the presence/absence of the corona from the hardness/softness of spectra during the low accretion phase. Additionally, we measure black hole masses, variability timescales, and the prominence of their coronal/disc emission by studying the effect of the different modes of AGN accretion on line-emitting gas. We compare the results of this study with those of our previous X-ray studies on moderate accretion rate AGN selected from the BAT AGN Spectroscopic Survey sample.

I present the analysis of a sample of several hundred SDSS quasars with multiple serendipitous XMM-Newton observations. The X-ray to UV luminosity relation allows to predict the average X-ray flux, and to select only the X-ray observations that are deep enough to remove any bias towards higer-than-average flux states. The optical/UV SDSS spectrum allows to investigate the relation between X-ray variability and the total luminosity, the black hole mass, and the Eddington ratio. Considering that the optical/UV and X-ray observations are not correlated, I conclude that most of the "intrinsic" dispersion of the X-ray to UV relation is due to X-ray variability.

We present the first X-ray polarimetric measurements of three Seyfert 1 galaxies with IXPE, the NASA/ASI mission operating as of December 2021. The results allow us to directly constrain the geometrical shape of the hot corona for the first time. We discuss the implications for the physical interpretation of X-ray variability in these sources.

Reverberation Mapping (RM) is a powerful technique for studying the geometry and kinematics of the broad-line regions in AGNs, as well as measuring the masses of supermassive black holes. This is achieved by observing the delayed response of broad emission lines with respect to the varying continuum. Significant progress has been made over the past decade, with the accumulation of RM data in different emission lines of various types of AGNs, as well as the development of more sophisticated analysis methods. In this talk, I will try to review the past 10 years of RM of emission lines in AGNs and discuss future prospects.

With VLTI/GRAVITY and near-infrared (NIR) interferometry, we can directly spatially resolve the broad-line region (BLR) to probe its physics and derive supermassive black hole (SMBH) masses via dynamical modelling. This method provides an independent test of the assumptions of reverberation mapping (RM), which has been the main method used so far to study the small scales associated with the BLR. In this talk, I will present our study of 7 type 1 AGNs observed with VLTI/GRAVITY. All of our studied BLRs can be well described by a thick, rotating disk of clouds. For each individual AGN, though, we can trace substructure and non-circular motions. For Mrk 509 and PDS 456 in particular, we find evidence for significant outflows. Interestingly, we find significant spatial offsets between average photocenters of the hot dust continuum and the BLR (ranging from $\sim$17$\mu$as to 140$\mu$as), which seem to follow a tight relationship with the AGN luminosity. I will discuss our interpretation of this relation, together with the implications of our results with RM, the physics of the BLR, and scaling relations such as the radius-luminosity (R-L) and black hole mass – stellar velocity dispersion ($M_{\rm BH}$ - $\sigma_*$) relations.

Reverberation mapping (RM) of Active Galactic Nuclei (AGNs) is the primary method to measure AGN broad line region (BLR) sizes and black hole (BH) masses. Most objects in the current H$\beta$ RM sample are low-to-intermediate luminosity AGNs with only a few objects having $L_{5100}\geq10^{44.5}$ erg/s. Here we present the latest results from our 6-year Seoul National University AGN Monitoring Project (SAMP). With hundreds of nights of regularly sampled spectroscopic/photometric observations, we successfully obtain reliable H$\beta$ lags and BH masses for 24 objects in the luminosity range of $L_{5100} = 10^{44.1\sim45.6}$ erg/s. The BLR sizes of these objects are generally smaller than the expectation from Bentz et al. relation. By applying an uniform lag analysis to literature H$\beta$ RM light curves and selecting reliable lag measurements to combine with SAMP measurements, we find the current H$\beta$ size-luminosity relation has a slope of $0.41\pm0.02$ with an intrinsic scatter of 0.19 dex. We confirm that the accretion rate / UV-optical spectral energy distribution is related to this shallower slope. In addition, we will present the H$\beta$ velocity resolved lag measurements for $\sim20$ AGNs and discuss the implication of these results on the BLR properties.

By long-term spectroscopic and photometric monitoring of three luminous intermediate-redshift quasars (CTS C30.10, HE 0413-4031, HE0435-4312) by the SALT telescope, we have been able to progressively constrain the parameters of the MgII radius-luminosity (R-L) relation (Czerny+2019, Zajacek+2020, Zajacek+2021). The MgII line variability is comparable to the continuum variability and the MgII light curve is significantly correlated with respect to continuum light curve. Thanks to that, we were able to infer the rest-frame time delays of the MgII emission, which are 276, 303, and 296 days for CTS C30.10, HE 0413, and HE0435, respectively. In combination with SDSS and OzDES reverberation mapping programs, which monitored lower-luminosity sources, the MgII R-L relation is constrained well using 94 sources up to now. The luminous quasars are crucial for enhancing the R-L correlation. The MgII R-L relation has a large vertical scatter of ~0.39 dex and a slope of ~0.3, which is in tension with the simple photoionization theory. We compare the MgII R-L relation with optical Hbeta, optical and UV FeII relations. The flatter slope of MgII R-L relation with respect to other R-L relations could be caused by the bias towards higher-Eddington, intermediate-redshift sources whose time delays are shortened for a given luminosity. In addition, we show that MgII R-L relation parameters are independent of the adopted cosmological model, and thus the monitored quasars can be standardized and applied for constraining cosmological parameters of different models (flat and non-flat cosmological models with general dynamical dark energy). Inferred cosmological constraints are weak but consistent with better established cosmological probes.

We present the analysis of a five nearby AGN that present extended emission line regions (EELRs) observed with the VLT/MUSE spectrograph. Spatially resolved emission line diagnostics indicate that the EELRs have been primarily photo-ionized by their AGN. The stellar and gas component kinematics indicate past merger or galaxy interactions that have perturbed all of these sources.
We generate sets of photo-ionization models and fit these to different regions along the different EELRs, covering distances of tens of kpc from the centre. These models allow us to estimate the bolometric luminosity required at different radii to excite the gas at the observed state. Our results suggests a systematic gradual decrease in AGN luminosity, and hence the accretion rate onto the SMBH, by a factor ∼ 100 over the past ~ 10^4 yr for every galaxy in the sample. This allow us to probe AGN variability on scales larger than possible for human timescales.

PKS 2004-447 is a narrow-line Seyfert 1 (NLS1) galaxy harboring relativistic jets capable of producing gamma-ray emission. On 2019-10-25, the Fermi Satellite detected a gamma-ray flare from this source for the first time. Thanks to coordinated spectral observations, we had a unique opportunity to study the behavior of the broad-line region (BLR) during a jet flare and searching for optical variability. Despite the obvious importance of understanding whether the jet can interact with the BLR, this aspect has not been thoroughly investigated. In my talk, I will introduce the peculiar nature of PKS 2004-447, which has remained poorly understood since its identification more than twenty years ago. I will also report on the results of our FORS2 and X-Shooter observations carried out before, during, and after the flare. During the high-energy event, a flux excess redshifted by 250 km/s is clearly seen in the Balmer, Paschen, and He I permitted lines. Such behavior has never been observed before, and interestingly this new emission feature is no longer visible 1.5 years after the flare, indicating a possible causal connection with the gamma-ray flare. The emission lines coming from the same atomic transition series show a similar velocity offset for this "red excess", but the offset changes for different line series. This discovery suggests that the relativistic jet can affect the physics of the BLR in this peculiar AGN, and that flaring activity can lead to the formation of additional and localized broad emission components. Our results highlight the importance of optical spectroscopy for flaring jetted AGN, and that our understanding of the jet-BLR connection is still very limited. These results will be used as a starting point for future dedicated studies of this kind.

Traditional accretion disk models have always had problems explaining a variety of observed features of AGN, particularly the short wavelength SED in the ultraviolet and beyond, and the rapid variability. I will review possible resolutions to these problems, including the effects of outflows, opacity-driven convection in the disk, and magnetically elevated disks.

Sgr A$^*$, located only 8 kpc away, allows us to study in detail the accretion process on to a super-massive black hole. Direct observations show that the black hole luminosity varies on different time-scales, but remains extremely dim, despite the (disputed) presence of a cold gaseous disc. However, indirect evidence reveals that it was several orders of magnitude brighter just a few hundred years ago, and perhaps an AGN a few million years in the past. Unlike any other super-massive black hole, in our Galactic centre we can directly observe the source of the material feeding the accretion, which in this case corresponds to a few dozen young, massive stars, with powerful stellar winds. After reviewing the observed variability, I will describe our hydrodynamical models of the gas surrounding Sgr A$^*$, originating from the observed stars, with known orbits and stellar wind properties. Our simulations show that these winds can naturally account for the formation of both the hot, inefficient accretion flow and the cold disc. Moreover, the stellar orbits, and the formation of cold clumps and streams, make the accretion vary on time-scales of decades to millennia, potentially explaining the observed behaviour.

Radio variability in some radio-quiet (RQ) active galactic nuclei suggests emission from regions close to the central engine, possibly the outer accretion disc corona. If the origins of the radio and the X-ray emission are physically related, their emission may be temporarily correlated, possibly with some time delays. We present the results of quasi-simultaneous radio and X-ray monitoring of three RQ Seyfert galaxies, Mrk 110, Mrk 766, and NGC 4593, carried out with the Very Large Array at 8.5 GHz over a period of about 300 days, and with the Rossi X-ray Timing Explorer at 2-10 keV over a period of about 2000 days. The radio core variability is likely detected in the highest resolution (A configuration) observations of Mrk 110 and NGC 4593, with a fractional variability amplitude of 6.3% and 9.5%, respectively. A cross-correlation analysis suggests an apparently strong (Pearson $r = -0.89$) and highly significant correlation ($p = 1 \times 10^{-6}$) in Mrk 110, with the radio lagging the X-ray by 56 days. However, a further analysis of the $r$ values distribution for physically unrelated long time delays, reveals that this correlation is not significant. This occurs since the Pearson correlation assumes white noise, while both the X-ray and the radio light curves follow red noise, which dramatically increases the chance, by a factor of $\sim 10^3$, to get extremely high $r$ values in uncorrelated data sets. A significantly longer radio monitoring with a higher sampling rate, preferably with a high-resolution fixed radio array, is required in order to reliably detect a delay.

Active Galactic Nuclei (AGNs) can collect stars and stellar remnants from the vicinity of the galactic center into the inner plane of the AGN disk. The dense population of stellar objects give rise to a wealth of interactions from stellar-mass black hole collisions to the tidal disruption of stars on stellar-mass black holes. These transients are promising multi-messenger sources from gravitational waves to radiation across the electromagnetic spectrum. I will discuss what we currently know about AGN-assisted mergers and disruptions so far, and highlight the most promising future observational directions.

Apart from regular, low-level stochastic variability, some AGNs occasionally show exceptionally large changes in luminosity, spectral shape, and/or X-ray absorption. The most notable are the changes of the spectral type when the source classified as a Seyfert 1 becomes a Seyfert 2 galaxy or vice versa. Thus a name was coined as 'Changing-Look AGN' (CL AGN). The origin of this phenomenon is still unknown, but for most of the sources, there are strong arguments in favor of intrinsic changes.

Understanding the nature of such rapid changes is a challenge to the models of black hole accretion flows since the timescales of the changes are much shorter than the standard disk viscous timescale, related to changes in angular momentum distribution.
We aim to model the CL AGN phenomenon using the time-dependent evolution of a black hole accretion disk unstable due to the dominant radiation pressure. We use a 1-dimensional, vertically integrated scheme, and focus on the variability timescales and amplitudes, which can be regulated by the action of large-scale toroidal magnetic fields and the presence of an inner optically thin flow, like Advection-Dominated Accretion Flow (ADAF). We thus modify the inner boundary condition of the cold disk flow, and we mimic the formation of the MRI-inactive zones, that suppress instabilities, by parameterizing their relative importance according to a local accretion rate. We succeed to model the timescales of tens of years that correspond to timescales of observed repetitive outbursts in CL AGN, such as NGC 1566 or NGC 4151.
However, other interpretations of quasar variability are still open and most probably more than one mechanism is responsible for changes observed in CL AGN.

Variability characterizes AGN at all wavelengths and is observed both in continuum and line emission. My talk aims at giving an overview of AGN optical/UV variability, focusing on the main results and challenges from the past decades.

Variability of AGN in all wavelengths has been known for decades, with timescales ranging from days to years. However, the physical mechanisms driving such variability are still unclear. X-ray Power Spectral Densities (PSDs) are usually well represented by power laws with slopes α ~ -1 at low frequencies, and α ~ -2 at high frequencies. Similar power-law trends have also been observed in UV/optical bands, but with a much lower break frequency. Optical variability is typically studied through Structure Function (SF) and modeled with a Damped Random Walk (DRW), implying a PSD with slopes at low and high frequencies, α=0, α=-2, respectively. Despite the good agreement of the DRW model on timescales from several months to a few years, many works show significant deviations on both longer and shorter timescales, along with strong uncertainties in determining the position of the break.

I will present a completely model independent study of AGN optical variability through ensemble PSD analysis on archival data. The wealth of information about bolometric luminosities and black hole masses enable the study of correlations between the variability amplitude and the AGN physical properties. PSD also has the advantage that its estimates at different frequencies are uncorrelated and with well known statistical properties. Moreover, as X-ray variability is usually studied through PSDs, using the same tool for optical bands provides better constraints on different variability models. Results from this analysis will be further boosted by the upcoming Legacy Survey of Space and Time (LSST), which will increase both the size of the sample and the temporal baseline, compared to previous surveys.

All quasars show a common stochastic variability, seen across various observed wavelengths and timescales. The origin of this variability is still uncertain, though variability in the optical is thought to stem from processes in the accretion disk around the SMBH. Time-series variability analysis presents a unique way to probe a quasar's geometry and dynamics in this regime without ultra-fine spatial resolution.

Optical quasar variability has been shown to be well-described by the Damped Random Walk (DRW) model, which is parameterized by a characteristic timescale $\tau_{\rm DRW}$ and amplitude $\sigma_{\rm DRW}$. A set of these parameters for a sample of quasars can be used to describe the variability statistically, which can then be related to physical properties of the accretion disk and its SMBH. For example, it has been shown that $\tau_{\rm DRW}$ correlates with the SMBH mass $M_{BH}$.

To investigate the validity and bias of the DRW model, in a recent work, we perform DRW-fitting analysis on multi-band 20-year-long optical quasar light curves for a sample of nearly 200 quasars, the longest baseline so far in DRW analysis.

We find that many of the timescales are still biased, though they are becoming less biased as baselines increase. We also find, using more flexible models, that the group (i.e., ensemble) power spectrum of the sample differs from the theoretical DRW model power spectrum on short timescales ($<$ a month), though matches it on longer timescales ($\sim$ months to years).

I will discuss the future of DRW and variability studies, involving reverberation mapping projects and correlations to certain timescales and instabilities within the disk. I’ll also mention the help that future surveys and programs (e.g., LSST and DECam) will provide in this type of analysis.

I present initial results from long-term (U)BV(u')g'r' photometry with the Las Cumbres Observatory robotic telescope network of a sample of ~80 AGN with a cadence of typically 1 month. The sample includes multiple representatives from the following AGN sub-categories: NLS1 with strong Fe II emission; Seyferts with Keplerian rotator broad line profiles; Seyferts with strong broad He II emission; obscured AGN; known Changing-look AGN; blazars. I utilise the flux variation gradient (FVG) method to determine the colour of an AGN's variable component. In most cases the FVG method also enables the separation of the variable and non-variable optical flux contributions and the estimation of the nuclear reddening. Since commencing this programme three years ago variations have been confirmed in >80% of the sample, and nuclear colours with an accuracy of 0.1 mag or better in B-V have been determined for half of the observed AGN. From these colours I determine the intrinsic variable component optical flux distributions and examine if and how these differ between the AGN sub-categories mentioned earlier. I briefly discuss how this constrains physical models of the variable parts of these AGN. Additional results include the indication that NLS1 with strong Fe II emission display lower optical variability amplitudes and that the reddening law for the dust obscuring some AGN differs to that applicable to typical interstellar dust.

Quasars optical variability gives us clues to understand the accretion disc around supermassive black holes. We can expect variability properties to correlate with the main physical properties of the accreting black hole, i.e., its mass and accretion rate. It has been established that the relative amplitude of optical variability anti-correlates with the accretion rate and luminosity.The dependence of the variance on black hole mass has remained elusive, and contradicting results, including positive, negative, or no correlation, have been reported. In this work, we show that the key to these contradictions lies in the timescales of variability studied (e.g., the length of the light curves available). By isolating the variance on different timescales as well as mass and accretion rate bins we show that there is indeed a negative correlation between black hole mass and variance and that this anti-correlation is stronger for shorter timescale fluctuations. The behavior can be explained in terms of a universal variability power spectrum for all quasars, resembling a broken power law where the variance is constant at low temporal frequencies and then drops continuously for frequencies higher than a characteristic frequency f_b, where f_b correlates with the black hole mass.  Furthermore, to explain all the variance results presented here, not only the normalization of this power spectrum must anti-correlate with the accretion rate, but also the shape of the power spectra at short timescales must depend on this parameter as well. In this talk I will present the possible interpretations of the dependence of power spectral shape on both parameters as well as the data supporting the results.

I will briefly review basic analysis methods used to describe the typical optical variability of AGN - structure functions and power spectra. I will discuss the applicability, usage, biases, and limitations of these methods and present some of the results for the OGLE AGN 20-year-long sample.

Variability studies of Active Galactic Nuclei (AGNs) have proven to be a powerful diagnostic tool for understanding the physics and properties of these objects. They provide insights into the spatial and temporal distribution of the emitting regions, the structure and dynamics of the accretion disk, and the properties of the central black hole. Here, we present the results of a ten-year campaign to monitor the near-infrared emission of the Seyfert 1.9/2 nucleus of NGC4388, covering J and K band spectroscopy. During this period, the hot dust continuum of the nucleus of this object varied by up to 200% under certain wavelength ranges. However, emission lines of low and medium ionization did not change beyond our error margin, whereas we detected variations of almost 100% at the [Ca$_{\rm VIII}$] coronal line. These results suggest that between 2011 and 2013, we were able to access an unresolved nuclear region that became obscured after 2015. We also mapped continuum and emission lines beyond the nucleus and found no significant variation within this time frame. These maps also indicate that emission lines are distributed along two main directions, representing the disc and the radio jet. Furthermore, the ionization of the emission lines is compatible with photoionization by an AGN in our whole 8x8 arcsec² field of view. Lastly, we detected a strong decrease in dust reddening along the radio jet, suggesting that the AGN is destroying dust grains in this region.

We use data from the Panoramic Survey Telescope and Rapid Response System 1 Survey (Pan-STARRS1, PS1) to extend the Sloan Digital Sky Survey (SDSS) Stripe 82 quasar light curves. Combining PS1 and SDSS light curves provides a 15 yr baseline for 9248 quasars–five years longer than prior studies that used only SDSS. We fit the light curves with the damped random walk (DRW) model –  a statistical description of their variability. We correlate the resulting DRW model parameters (asymptotic variability amplitude, and characteristic timescale), with quasar physical properties (black hole mass, bolometric luminosity, and redshift). We also make predictions for the fidelity of DRW model parameter retrieval when light curves will be further extended with Zwicky Transient Facility (ZTF) and the Rubin Observatory Legacy Survey of Space and Time (LSST) data. Finally, we show how updated DRW parameters offer an independent method of discovering changing-look quasar candidates (CLQSOs). The candidates are outliers in terms of differences in magnitude, and scatter between SDSS and PS1 segments. We identify 40 objects (35 newly reported) exhibiting a tenfold increase in variability timescale between SDSS and SDSS - PS1 data. An accompanying large (over 0.5 mag) change in brightness is characteristic of CLQSOs. We summarize the results of a recent program of spectroscopic follow-up of select CLQSO candidates carried out at the Apache Point Observatory.

Intermediate-mass black holes (IMBHs) are key pieces in the puzzle of extragalactic and galactic astronomy, due to their potential to answer questions related the formation and evolution of supermassive black holes and co-evolution with their host galaxies, among others. Because of the difficulties present when detecting and confirming sources as IMBHs, they have proven to be an elusive population. Accreting BHs are known to show random variability in different spectral bands (optical, UV, etc.). We aim to demonstrate the viability of optical variability as a technique to select IMBHs candidates and characterize a sample of IMBHs obtained from the literature. Using ZTF forced photometry on the difference image, and various variability features, we obtain a high-confidence IMBHs candidates subgroup. We aim to study the multi-wavelength properties of the selected subsample and discuss it's implications in the AGN paradigm.

I am going to present the results of the search for small-amplitude (A_g > 0.03 mag), long-period (100 < P[days] < 600) variability in the SDSS Stripe 82 region. This search led to the discovery of five quasars with apparently periodic light curves. In addition, I will discuss the line profile variability (presumably linked to the change in the phase of the optical light curve) of the MgII emission line of our strongest periodically variable candidate quasar (P=278 days), obtained over the past year. With the data collected so far we were not yet able to exclude the possibility of the object being system of supermassive binary black holes.

Our search was made possible by the precisely calibrated (1%-2%) Stripe 82 photometry in SDSS ugriz bands, which covered a period of approximately 6 years and reached down to r~22 mag. By analyzing the Lomb-Scargle periodograms, we identified the most promising candidates for periodically variable sources. We then cross-matched these candidates with other surveys across the electromagnetic spectrum (photometry and spectroscopy) to confirm their variability and type. Our analysis was supported by Pan-STARRS and ZTF time series, which provided observational data spanning more than 20 years.

All of the identified candidates were quasars, and the highest-ranked one was flagged as a variable source in the Chandra X-ray catalog. The observed periodic behavior of quasars could be attributed to various factors, such as radio jet precession, tilted or warped accretion disks, tidal disruption events, and other accretion-related effects.

The Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST), aiming to begin in early 2025, will allow studies of the growing supermassive black holes (SMBHs) in active galactic nuclei (AGNs) on a truly massive scale. After a brief review of the LSST from an AGN perspective, I will describe the planned selection of tens of millions of AGNs using the LSST plus multiwavelength data, including with variability techniques. I will then highlight examples of exciting LSST AGN variability investigations including massive general AGN variability studies, photometric and spectroscopic reverberation mapping, microlensing of small-scale AGN structure, transient SMBH fueling events, and candidate binary SMBHs. I will end by briefly describing the LSST AGN Science Collaboration (AGN SC), currently composed of about 160 members spanning the globe. The AGN SC aims to lead many of the described investigations and is preparing for science with the petabyte deluge of LSST data.

The Zwicky Transient Factory (ZTF) has been surveying the visible sky above Dec = -30 on a 2-3 night cadence in $g$, $r$, and $I$ for the past five years. With a $5 \sigma$ detection limit of $g = 20.5$, this provides hundreds of thousands of nightly real-time public alerts as well as well-sampled light curves for over 3 billion sources. This is an unparalleled data set for both anomaly detection and population studies of many astrophysical classes but it has been particularly fruitful for supermassive black holes: both the tens of tidal disruption events found around quiescent systems but also detecting new types of phenomena in active systems. In this talk, I will review what insights ZTF is providing into the AGN Zoo with changing state quasars, ambiguous nuclear transients, and EM counterparts to stellar mass black hole mergers as well as the more general properties of the AGN population.

The third release of Gaia data, published on June 13, 2022, includes not only astrometric and astrophysical parameters of different types of sources, but also several catalogues of variable sources. Among these, the catalogue of Gaia variable AGN, which is described by Carnerero et al. (2022). To identify the variable AGN, we analyzed the light curves of more than 80 million sources observed by Gaia, selecting 870 thousands of them compliant with requirements on their variability properties (structure function, Butler and Bloom metrics, fractional variability), color indices, astrometric parameters and others. The purpose was to create a variable AGN catalogue as pure as possible. More than 21,000 of these 870 thousand sources are identified as variable AGN for the first time. For each source of the catalogue, the Gaia multi-band light curves and the values of a number of variability parameters have been included in the database. In view of the next Gaia DR4, we are now implementing new variability parameters that will be published in the Gaia AGN variability table. They will be useful to the community, both for carrying out statistical studies and for AGN classification with machine learning.

With about 380 refereed papers published each year, XMM-Newton is one of the most
successful scientific missions of ESA ever. Observation of AGNs and their variability is one of the main research fields covered by the observing program of the mission. The talk highlights XMM-Newton contributions to our current view of Black Holes variability. XMM-Newton observations provide a unique opportunity to study the vicinity of Supermassive Black Holes (SNBH) and constrain the understanding of the underlying accretion physics. The main focus of the talk will be the discussion of recent scientific highlight results based on XMM-Newton observations of SMBHs.

The La Silla QUEST (LSQ) supernova survey ran for 6 years on the ESO 1m Schmidt telescope at La Silla Chile, using a large CCD array to replace the photographic plate of the Schmidt. The survey imaged ~1000 degrees twice per night using a single broad V band filter, covering a total area of ~25,000 square degrees from declination ~ -80 to +25 degrees. The survey magnitude limit is V~21 in a single 60 second exposure, with an average of ~200 visits for any given patch of sky and over one thousand square degrees of sky covered by more than 1000 visits. Systematic photometric errors from the current differential photometry pipeline are at the 5-10 mmag level for bright point sources on a good night, with further improvements expected. The QUEST V filter can be absolutely calibrated against SDSS and PanSTARRS g+r data at the percent level, enabling LSQ, for example, to extend in time the SDSS Stripe 82 variability survey. Although principally designed to find supernovae, the strict survey cadence provides good logarithmic time coverage on timescales from ~30 minutes to ~years, ideal for probing AGN variability and constraining, for example, the parameters of a Damped Random Walk model. We present some highlights of LSQ AGN science. The LSQ dataset should prove useful as a training set to prepare for LSST. Over 1 million LSQ-selected AGN candidates will also be followed up by the upcoming ESO 4MOST survey.

eROSITA (extended ROentgen Survey with an Imaging Telescope Array), the core instrument on the Russian-German Spektrum-Roentgen-Gamma (SRG) mission has completed 4 scans of the entire sky with unprecedented sensitivity in the 0.2-8 keV energy range. I will present an overview of the instrument capabilities, the current status of the mission, a few selected early science results focusing on the study of the time domain properties of galactic nuclei.

The eROSITA all-sky X-ray survey has provided the basis for a large-scale search for extreme X-ray variability in extragalactic objects associated with accretion changes in AGN. We have combined the survey dataset with a multi-wavelength follow-up campaign of the most variable objects. The follow-up observations include optical spectroscopy and X-ray and UV observations. This presentation will cover the results of our search for extremely variable AGN based on at least four epochs of available eROSITA data. Our sample consists of ~2200 vetted extragalactic sources with significant X-ray changes. As part of our follow-up, we have collected optical spectroscopic follow-up on ~350 objects, including repeat spectroscopy for 40% of these. I will introduce our sample selection criteria, statistics on the detected X-ray variability, and the observed correlation with optical 'changing-look' behaviour. I will also briefly summarise some of the most interesting individual sources. Finally, I will discuss our results in the context of the link between extreme X-ray and optical variability and the time scales involved in large-scale accretion changes around SMBHs.

Recent years have seen broad observational support for the
circumnuclear gas around supermassive black holes to contain a clumpy
component. In the X-ray band, individual clouds can manifest
themselves when they transit the line of sight to the X-ray corona,
temporarily obscuring the X-ray continuum, and indicating the
characteristics and location of these clouds.

The eROSITA X-ray telescope aboard Spectrum X/Gamma is performing
multiple all-sky X-ray surveys, including monitoring a vast sample of
AGN and galaxies. Such monitoring can amplify rare cloud occultation
events, allowing us to accumulate observational constraints for
clumpy-torus models, including cloud distribution and composition
parameters.

Here, we discuss the first cloud occultation events detected in a
Seyfert 1 galaxy by eROSITA: in this Seyfert, the soft X-ray flux
dipped abruptly for $\sim$ 10-18 months during 2020-2021, recovered,
but then dropped a second time by Spring 2022. Our two-year
multi-wavelength follow-up campaign included X-ray/UV and ground-based
optical photometric and spectroscopic observations, and confirmed that
the soft X-ray flux dips were caused by partial-covering obscuration
by two separate, single compact clouds near the black hole.
The two transiting clouds are consistent with neutral or lowly-ionized
gas, residing at radial distances commensurate with the optical Broad
Line Region and the inner dusty torus, respectively.

The eROSITA all-sky surveys (eRASS) continuously scan the sky
along great circles crossing the ecliptic poles. This scanning strategy
covers the full sky every six months and visits the ecliptic poles every
four hours, leading to much longer exposure time and much higher cadence
at the ecliptic poles than the majority of the sky. Between Dec. 2019
and Feb. 2022, the eRASS surveys scanned the full sky more than four
times and observed the ecliptic poles more than 4600 times, with a total
exposure of more than 160ks at the ecliptic poles. Because of the long
exposure near the confusion limit of eROSITA, we treated the region
within 3 degrees of the south ecliptic pole (SEP) separately from the
main part of the eRASS surveys and detected X-ray sources with a
pipeline fine-tuned for such crowed fields. We built a catalog with ~15k
X-ray sources within 3 degrees of SEP (~8k within 1 degree) and
identified their multiband counterparts from a few catalogs including
CatWISE, NSC-DR2, S-CVZ, and GAIA-DR3. Based on multiband colors, we
selected AGN from them and used the AAOmega spectrograph onboard the AAT
telescope to follow them up. A large number of the X-ray sources exhibit
significant variability, including a few particular targets that are
possibly tidal disruption events, AGN shutting down or ignition events,
or quasi-periodic oscillation cases. We study the normalized excess
variance and power spectral densities of AGN with X-ray light curves,
measure their optical properties, e.g., black hole mass, using optical
spectra, and analyze the correlation between them.

The Survey and Time-domain Astrophysical Research eXplorer (STAR-X;
http://star-x.xraydeep.org) is a Medium Explorer class mission
recently selected for a competitive NASA Phase A study. It comprises a
wide-field, high-throughput, high-angular-resolution X-ray Telescope
(XRT) and a complementary UV Telescope (UVT) on an agile spacecraft
bus. STAR-X will conduct high-cadence, deep-and-wide surveys, and
respond rapidly to transient events discovered by other observatories
such as LIGO, Rubin/LSST, Roman/WFIRST, and SKA.

The science theme for the mission is “to study the fast, furious and
forming Universe.” In this talk I will first present an overview of
the mission concept and observing capabilities, and then focus on the
key "furious" science pillar, which will explore feeding and growth of
massive black holes through sensitive, time-domain studies.

STAR-X will uniquely probe the physics of rapid accretion that allowed
the formation of the first supermassive black holes, and will catch
transient, extreme black hole feeding events, such as Tidal Disruption
Events (TDEs). Critically, STAR-X will discover TDEs in the X-ray
band, providing direct evidence for newborn accretion disks. Also, by
monitoring their X-ray and associated UV emission, STAR-X will
constrain the timescales of disk formation and their
evolution. Finally, STAR-X will perform detailed reverberation mapping
of AGN distributed over a broad range of Eddington ratios, revealing
how the accretion flow geometry depends on the accretion rate.

AGN are known to show flux variability over all observable timescales and across the entire electromagnetic spectrum. Over the past decade, a growing number of sources have been observed to show dramatic flux and spectral changes, both in the X-rays and in the optical/UV. Such events, commonly described as “changing-look AGN”, can be divided into two well-defined classes. Changing-obscuration objects show strong variability of the line-of-sight column density, mostly associated with clouds or outflows eclipsing the central engine of the AGN. Changing-state AGN are instead objects in which the optical/UV continuum emission and broad emission lines appear or disappear, and are typically triggered by strong changes in the accretion rate of the supermassive black hole. In my talk I will review our current understanding of these objects, and then focus on a few recent X-ray monitoring campaigns of Changing-state AGN.

A growing number of transient phenomena in galaxy nuclei have recently begun to shed new light on SMBH demographics and the physics of gas accretion onto these objects, tracing events where this accretion has drastically intensified, diminished, and/or otherwise disturbed. I will present recent results regarding some of these new classes of high-variability phenomena, focusing on insights gained thanks to responsive, multi-wavelength follow-up observations. These include “changing look” AGN that occur on surprisingly short timescales (several weeks), and for which we have strong evidence for the nature of the transition (i.e., accretion vs. obscuration); and other, yet poorly understood flaring AGN with broad Bowen fluorescence emission features, driven by extreme UV radiation that appears within weeks but lasts for well over a year. While these events observationally differ from the tidal disruption events known to date, the physics behind them may be interlinked. Together, these extreme events can greatly advance our understanding of SMBH accretion, teach us how and why SMBHs turn their accretion “on” and “off”, and reveal super-Eddington accretion. I will finally mention how new surveys, such as the SDSS-V, will discover & survey many more SMBH-related transients.

Changing-look (CL) AGN are unique probes of accretion onto supermassive black holes (SMBHs), especially when simultaneous observations in complementary wavebands allow investigations into the properties of their accretion flows. I will present the results of a search for CL behaviour in 412 Swift-BAT detected AGN with multiple epochs of optical spectroscopy from the BAT AGN Spectroscopic Survey (BASS). 125 of these AGN also have 14-195 keV ultra-hard X-ray light curves from Swift-BAT which are contemporaneous with the epochs of optical spectroscopy. We have discovered eight new CL events, where the appearance or disappearance of broad Balmer line emission leads to a change in the observed Seyfert type classification. Combined with known events from the literature, 21 AGN from BASS are now known to display CL behaviour. Nine CL events have 14-195 keV light curve coverage, and five of these CL events can be associated with significant changes in their 14-195 keV flux from BAT. The ultra-hard X-ray flux is less affected by obscuration and so these changes in the 14-195 keV band suggest that the majority of our CL events are not due to changes in line-of-sight obscuration, and instead must be due to changes in the structure of the accretion disk and broad line region. We derive a CL rate of 0.7-6.2 per cent on 10-25 yr time-scales, and show that many transitions happen within at most a few years. Our results motivate further multiwavelength observations with higher cadence to better understand the variability physics of accretion onto SMBHs.

Recent advances in time-domain surveys have revealed dramatic changes to SMBH accretion and AGN appearance on surprisingly short timescales. Among those, changing-look AGNs (CL-AGNs) show the (dis)appearance of broad emission lines and/or the quasar-like continuum, on timescales of years and sometimes even months. These dramatic changes may be driven by significant changes to the accretion flow and/or circumnuclear gas, and can therefore provide key novel insights into these physical components. In this talk I will present the largest sample of (candidate) CL-AGNs to date, with >100 sources, obtained from the first year of observations by the recently launched SDSS-V project, and assisted by follow-up observations with HET, Palomar, and LCOGT. Our sample covers a redshift range of 0.06 < z < 2.5 and rest-frame transition timescales lasting from 2 months and up to 19 years. Our preliminary analysis shows that CL-AGNs occur at systems with relatively low Eddington ratios, but with no preference for certain BH masses or luminosities. I will highlight a particularly extreme CL-AGN, varying on timescales of <2 months, whose optical spectrum and lightcurve are most likely driven by variable obscuration, on unprecedentedly short timescales. Our large sample will allow us to gain insights into the physical mechanisms of CL-AGNs, with potential implications for the unified AGN model and thus for AGN demographics.

We present spectroscopic and photometric observations of the changing look AGN
IRAS23226-3843. This object has previously been classified as a
changing-look AGN based on observations taken in the 1990s in comparison to
X-ray data (Swift, XMM-Newton, and NuSTAR) and optical spectra taken after
a very strong X-ray decline in 2017. In 2019, Swift observations revealed
a strong rebrightening in X-ray and UV fluxes. We took follow-up X-ray
observations of IRAS23226-3843 together with optical spectra from 2019 until
2021. IRAS23226-3843 showed a strong X-ray and optical outburst in 2019. It
varied in the X-ray continuum by a factor of 5 and in the optical continuum by
a factor of 1.6 within two months. The Balmer and FeII emission-line
intensities showed comparable variability amplitudes during the outburst in
2019. The Halpha emission-line profiles of IRAS23226-3843 changed from a
blue-peaked profile in the years 1997 and 1999 to a broad double-peaked
profile in 2017 and 2019. However, there were no major profile variations in
the extremely broad double-peaked profiles despite the strong intensity
variations in 2019. One year after the outburst, IRAS23226-3843 changed its
optical spectral type and became a Seyfert type 2 object in 2020.
A deep broadband XMM/NuSTAR spectrum was taken during IRAS23226-3843 maximum
state in 2019. This spectrum is qualitatively very similar to a spectrum taken
in 2017, but by a factor of 10 higher. The soft X-ray band appears featureless.
The soft excess is well modeled with a Comptonization model.

Tidal disruptions of stars by supermassive black holes were proposed in the 1970s as a possible way of fuelling active galactic nuclei. Following further studies showing that this mechanism can not supply quasar-level fuelling rates, it was realized that bright flares produced by disruptions could be used as probes for exploring otherwise quiescent galactic centres.
In the last decade the study of Tidal Disruptions Events (TDEs) gained momentum from advanced numerical simulations on the theoretical side and from detections of dozens of TDEs by wide-field sky surveys on the observational side.

I will review the theoretical picture of TDEs, main mechanisms and parameters affecting the outcome of a stellar encounter with a black hole, and their observed characteristics. I will address main open questions stemming from simulations and observations, and the prospects of Rubin Observatory LSST in discovering TDEs, and significantly enlarging their sample.

Tidal disruption events of stars provide unique opportunities for probing massive black holes in centers of galaxies. Furthermore, these events are great laboratories for studying black hole accretion and outflow physics. In this talk, I will first give a theoretical overview of the accretion, wind and jet physics tidal disruption events. Then I will discuss our latest discoveries on using general relativistic magnetohydrodynamic and radiative transfer simulations to understand the accretion disk and outflow in tidal disruption events and connect theory to the observed signatures.

The disruption and subsequent accretion of a star by a super-massive black hole (SMBH) provides an excellent laboratory to study a broad range of accretion conditions over timescales as short as months or a few years. We show how a physical model of a relativistic thin accretion disc, applied to the X-ray spectra of a sample of 19 tidal disruption events (TDE) in the high-accretion thermal phase, can yield the black hole mass and inner radius of the disc. From this study we offer a possible solution for the problem of low apparent total mass accretion in TDE systems and perform a sanity check on the hypothesis that the peak optical/UV emission in TDEs is due to the reprocessing of X-ray radiation.

When the accretion rate in a TDE drops well below the Eddington limit, the spectrum is commonly observed to develop a hard X-ray tail, believed to be due to the Compton up-scattering of disc photons by a warm electron cloud. We show that
this component develops very rapidly in at least one source and use this to constrain the mechanism responsible for the creation of the Comptonisation zone.
Finally we present new data on a further interesting extra-galactic, hard X-ray transient, providing strong support that it was also caused by a TDE.

During its first two years of the All-Sky Survey, SRG/eROSITA uncovered a large sample of X-ray transients associated with the nuclei of quiescent galaxies. In this talk, I will highlight one exceptional repeating nuclear transient eRASSt J045650-203751 discovered by SRG/eROSITA. Extensive monitoring with XMM-Newton, Swift, NICER, and ATCA revealed four repeating X-ray flares and repeating transient radio emission. This makes J0456-20 one of the most promising repeating partial Tidal Disruption Event (pTDE) candidates. A detailed analysis of the available data shows that the characteristic X-ray variability for each flare can be best explained by the accretion state transitioning between the thermal and the steep power-law states, accompanied by the formation and destruction of the coronae. This indicates that similar accretion processes are at work across a broad range of BH masses and accretion rates and that the corona can be formed and destructed within a few weeks to months. I will also present evidence of a potential evolution of the recurrence time of the flares, hinting at a change in the orbital period of the stellar remnant. This highlights the role of repeating pTDEs as effective probes of the stellar dynamics around supermassive BHs beyond our Galaxy.

When stars approach the tidal radius of a supermassive black hole (SMBH) and find themselves unraveled, the resulting debris stream spirals toward the SMBH and creates a flare whose light can outshine the host galaxy. These tidal disruption events (TDEs) can be used for independent measures of black hole masses, and they offer new windows to study accretion onset and flaring mechanisms near SMBHs. Only recently, though, have TDEs offered us glimpses into the sub-parsec local environments near SMBHs. AT 2020mot is a typical UV/optical TDE, but is uniquely bright in the near-infrared and even shows a later enhancement in brightness along the tail of the light curve. This could be the first TDE to show two "dust echoes," indicative of concentric rings of thin dust within 0.1 parsecs of a SMBH, among the smallest scales at which dust has been inferred near SMBHs. Similarly, the recent event AT 2022upj is an extreme coronal line emitter (ECLE) that shares emission line diagnostics in common with the small subset of ECLEs designated as TDEs. This class of events has been interpreted as another form of a "light echo" of TDEs in gas-rich environments. Events like AT 2020mot and AT 2022upj are novel opportunities to peer into the closest material of otherwise invisible black holes in quiet galaxies. Studying these events will explore the fundamental connections between supermassive black holes, galaxy evolution, and accretion mechanics.

Serendipitously discovered at the end of 2018 in the nucleus of the galaxy GSN 069, X-ray quasi-periodic eruptions (QPEs) are a new extreme-variability phenomenon associated with supermassive black holes. QPEs typically appear as sharp and intense burst of X-ray emission, with a thermal-like spectrum with a temperature $kT \sim 150$ eV over a much more stable and cooler quiescent level; they last about one hour and repeat quasi-periodically every few hours, carrying a few $10^{42-43}$ erg s$^{-1}$ at each burst. X-ray QPEs have been securely detected in the nuclei of several galaxies since their discovery, all with low-mass supermassive black holes ($M_{BH} < 10^7 M_{\odot}$) and different levels of nuclear activity. In this talk I will review the general properties of QPEs and their variegate phenomenology in the QPE-sources (up to 7) detected so far. I will then discuss the physical scenarios invoked to explain this new and puzzling extreme-variability X-ray phenomenon, as well as the more and more evident physical connection of QPEs with tidal disruption events (TDEs) in low-mass galaxies.

We present the photometric and spectroscopic analysis of a new nuclear transient AT2020nov, an event that shows properties consistent with both TDEs and active galactic nuclei (AGN). Observations in the X-ray show late-time flaring, coincident with a minor re-brightening in the optical/UV. Evolution in the X-ray hardness ratio follows a trend from hard to soft, suggesting a change in the accretion behavior with time. Optical spectroscopy taken both before and after the light curve peak show a blue continuum, with resolved double-peaked Balmer emission and possible Bowen Fluorescence features. The discrepancy between the X-ray and UV/optical photometry indicates that the radiation sources are initially uncorrelated, with emission arising from physically distinct components. This implies a scenario in which the optical/UV peak is powered by collisions in the debris streams of a circularizing accretion disk, while the late-time X-ray and optical/UV bump result from the enhanced accretion rate of the circularized disk. However, modeling of the double-peaked Balmer features in the spectra with an elliptical accretion disk indicates that the disk formed early and fast, inconsistent with circularization occurring at late times. In this talk, I'll discuss how this new event fits into the landscape of nuclear transients generally, and TDEs/AGN specifically.

Despite the increasing number of newly discovered changing look active galactic nuclei (AGN), larger samples of known objects and multi-epoch observations are needed to shed light on this debated physical mechanism. In this talk, I will report on the changing look AGN in the galaxy NGC4156, as serendipitously discovered thanks to data acquired in 2019 at the Telescopio Nazionale Galileo (TNG) during a student observing program. Unlike previous optical spectra showing no signatures of broad-line emission, our 2019 TNG data unexpectedly revealed the appearance of broad components in both Hα and Hβ profiles, along with a rising continuum, overall pointing to a transition from a type 2 towards a (nearly) type 1 AGN. The broad-line emission has been then confirmed by our 2022 TNG follow-up observations, whereas the rising continuum is no longer detected, which hints at a further evolution backwards to a (nearly) type 2. I will discuss possible mechanisms at the origin of the observed optical variability of NGC4156, and briefly compare it to what observed in its X-ray multi-epoch observations. Approved optical (Asiago telescope) and X-ray (Swift) monitoring programs will provide us further insights into the variability of this source, possibly constraining the typical timescales of these changing look events.

Here we present our study of the variability of the broad Hbeta line profile of the "changing look" active galactic nucleus (CL-AGN) NGC 3516 over a long period (from 1996 to 2021).
We model the broad line profiles assuming that there is emission from the accretion disc superposed with emission from a surrounding region that is outside the disc.
We find that in the Type 1 activity phase (i.e., when the strong broad emission lines are observed), the broad line region (BLR) is very complex. There is a clear disc-like BLR contributing to the broad line wings and an additional intermediate line region (ILR) contributing to the line core. In the high activity phase, the ILR emission is close to the center of the line (slightly shifted to red in some cases), whereas in the low activity phase (i.e., Type 2 phase), the ILR component is clearly shifted to blue, indicating outflow.
At different activity stages, the complex BLR structure can be detected, indicating that the gas motion remains constant but the line emission becomes weak. This may be caused by dust entering the interior of the BLR during the low activity stage, forming a dusty BLR. This leads to a decrease in ionization and recombination rates, so that the broad lines almost disappear.

Mrk 1018 is an extremely unique changing-look AGN, which has already changed type twice. Almost a decade ago, it returned from a Seyfert type 1 to its original classification of a Seyfert type 1.9. We have been monitoring Mrk 1018 in the u’-band with STELLA since this last major transition. In 2020, our long-term optical monitoring program detected the most significant outburst over the last few years. With a flux increase of a factor ~13, this outburst alone would have flagged Mrk 1018 as a changing-look AGN in photometric searches. The outburst is asymmetric in the u’-band with a rise of ~100 days and a decline of ~200 days. It was confirmed by the ATLAS forced photometry server. Using both STELLA and ATLAS, we compared the outburst as seen in three optical wavebands. We also followed up with an extensive multi-wavelength dataset in X-ray, UV, optical and infrared to compare the AGN components before and after outburst. Optical spectra were taken approximately one year before and after the outburst and showed no change. X-ray and UV observations were taken 6 - 7 months before and after. The primary X-ray flux returned to the state before the outburst but the 6.4 keV iron line increased in strength and UV emission was also increased. The IR light curve responded to the optical outburst extremely quickly. The optical light decay is best described by a linear decline, indicating that the increase was not caused by a tidal disruption event of a star. I will summarise a recently submitted paper on this outburst in 2020, including speculation as to why why Mrk 1018 changes its energy output repeatedly and in such a drastic manner.

Supermassive black hole binaries are a natural end product of galaxy mergers and should be common in galactic nuclei. They produce bright electromagnetic emission and can be identified as quasars with periodic variability in time-domain surveys. They are also promising sources of low-frequency GWs soon to be detected by pulsar timing arrays (PTAs) with PTAs and time-domain surveys probing the same population of binaries. I will discuss the combination of time-domain observations with PTA data in a multi-messenger stream, the parameter space of binaries for which this combination is possible and the advantages of multi-messenger observations (e.g., improved parameter estimation).

Supermassive black hole binaries are thought to be an inevitable product of the prevailing galaxy evolution scenarios where most massive galaxies host a central black hole and undergo mergers over cosmic time. The early stages of this process have been observed in the form of interacting galaxy pairs and widely separated dual quasars, but the close, gravitationally bound binaries that are expected to follow have so far eluded observation. The detection of this population is important because at the smallest separations they become bright sources of low-frequency gravitational waves and are prime targets for multi-messenger detections. One approach to search systematically for close supermassive black hole binaries among quasars is based on the hypothesis that the secondary black hole in the system is feeding and the resulting emission lines will be doppler shifted due to its orbital motion. Binary candidates identified via this method are therefore selected from nearby quasars via substantial (>1000 km/s) shifts of the broad H-beta lines relative to the systemic redshift. One key test of this search is an ongoing spectroscopic monitoring campaign to look for signs of bulk motion of the quasar indicative of orbital motion. I will describe the observational research program that I have been leading, including our most compelling candidates and efforts to evaluate the credentials of these candidates in the face of quasar variability.

Dual super massive black holes at sub-kpc to kpc scales are the products of galaxy mergers and the progenitors of eventually coalescing binary SMBHs. Dual AGNs or off-nucleus AGNs may be witnessed if both or one of the dual SMBHs are accreting. Despite its rarity, such systems are essential for learning the dynamical evolution of binary SMBHs as well as the process of galaxy merging. Recently a novel and highly efficient astrometry-based method named varstrometry has been put forward to search for dual SMBHs at high redshift. This method shows that the unsynchronized flux variability of off-nucleus and dual AGNs will cause astrometric jitter detectable by Gaia without spatially resolving them. Based on varstrometry we select a rare sample of 5 high redshift radio quasars with clear Gaia astrometric jitters, and with e-MERLIN observations a single compact radio source is revealed for each of them. Clear Gaia-radio offsets of $\sim$ 9 -- 60 mas are deteced in all but one targets. The observed Gaia jitters appear consistent with the expected values. These detected Gaia-radio offsets suggest these candidate dual SMBHs may have projected separations as small as $\sim$ 0.01 - $0.1''$ ($\sim$ 0.1 kpc, depending on the optical flux ratio of two SMBHs).

PKS 2131−021 is a blazar that shows peculiar variability in the radio light curve: within 45 years of recorded data, two epochs show strong sinusoidal variation with roughly the same period and phase, straddling a 20 year period when this variation was absent. We apply the Lomb-Scargle periodogram, weighted wavelet Z-transform and least-squares sine-wave analyses and address two pitfalls that are commonly ignored in periodicity studies of blazars: First, blazar light curves typically exhibit red noise variability, which makes it necessary to employ a large set of simulated light curves that reflect such a process. Second, when no a priori knowledge about the signal period exists, the look-elsewhere effect needs to be taken into account over the tested frequency range. Our statistical analyses demonstrate conclusively, at the 4.6σ significance level, that the periodicity in this object is not due to random fluctuations in flux density. A simple model can explain the sinusoidal variability as a result of modulated Doppler boosting due to the orbital motion of a Supermassive Black Hole Binary (SMBHB). The observed period of ~2 years in the rest frame of the source suggests an orbital separation of ∼0.001–0.01 pc. If truly a SMBHB and sufficiently massive, the gravitational waves produced by this system may be detectable with future pulsar timing arrays.

Winds link the supermassive black holes at the heart of active galactic nuclei (AGN) to their environment. Combined high-resolution UV and X-ray spectroscopy is a crucial tool to advance our understanding of the origin and role of these outflows in AGN. I present results from recent studies investigating the physical connection between different forms of outflows that have been found in AGN. I review the UV perspective of warm-absorber outflows, transient obscuring winds, and the ultra-fast outflows (UFOs) using spectroscopy with the Hubble Space Telescope (HST). By deciphering their variability, and mapping their ionization and kinematic structure, new insights are gained on the formation and driving of the AGN outflows.

It is clear that an important part of the AGN self sustenance, as well as its connection with the surrounding, is constituted by powerful outflows, detected in a large fraction of objects. Winds can be considered as the messenger in the communication between the AGN and the galaxy.
Different wind components co-exist in the same source, often with drastically different properties in terms of carried mass and energy.
In this talk I will review X-ray winds and their variability as a function of the central engine flux. Several methods are in place to extract important information on the physics and geometry of the outflowing gas. This in turn provide important diagnostic on AGN feedback.

We present the results of a South African Large Telescope (SALT) spectroscopic monitoring to study the time variability of C IV BALs in a sample of 64 quasars showing ultra-fast outflow (UFO) with v$_{outflow,max}$ > 15000 kms$^{-1}$ in their spectra. We also created a sample of non-BAL quasars from SDSS DR12 matched in redshift and luminosity. Our UFOs show more blueshift of CIV BEL than that of non-BALs in the control sample. The fraction of “highly variable” BALs (with fractional change in equivalent width, $\frac{\Delta W}{W}$ > 2) in our sample is considerably higher than that reported for the general BAL population. We find that the strength of variability increases with time, and for each source, SALT observations enabled us to look at the variability at different time scales ( from as short as < 0.5 years to longer time scales of > 7.5 years) in detail. We also show that the fraction of highly variable BALs increases with time, and for these BALs, the BAL strengthening time scale is found to be considerably shorter than the weakening time scales. We found no correlation between BAL variability and quasar properties such as black hole mass and Eddington ratio but found a moderate correlation with bolometric luminosity for time scales < 2 years. Based on the properties of C IV absorption, we find weak, high-velocity, shallow, and low-width BALs tend to show more variability. We also classified the BALs according to their absorption profile shape and found detached profiles at high velocities showing large variations irrespective of the strength of absorption. We conclude both the low-equivalent width and high-velocity nature of BALs are equally important for excess BAL variability. Interestingly our results suggest that the presence of a distinct BAL trough at lower velocities increases the chances of observing a highly variable UFO BAL if present. Finally, using photometric light curves, we show that the continuum flux variations may be responsible for the observed BAL variability in the majority of the sources where the EW of the BAL decreases as the continuum increases.

Line driving is a promising explanation for AGN winds as it provides both a launching mechanism and an explanation for the absorption and emission lines in spectra. As the community moves towards multi-wavelength and multi-epoch observations, our modelling of AGN systems must likewise follow suit to leverage these new capabilities. For line driving to be a viable acceleration mechanism two conditions must exist in the wind 1) The gas must be sufficiently, though not overly, ionized by X-rays, so that the gas can interact with the UV 2) The UV flux incident on the gas must be high enough to transfer sufficient momentum to overcome gravity. We present novel simulations of AGN disc winds using time-dependent, multi-frequency radiation hydrodynamics focusing on the problem of gas ionization, where we model both the X-ray and UV radiation fields. We consider a suit of models for gas/X-ray interactions and identify the conditions on scattering and absorption opacities where wind self-shielding can operate and allows line driving to launch winds.

Radio-loud AGN are characterized by plasma jets that are formidable particle accelerators. In blazars we observe jets at a small angle with respect to the line of sight, with consequent relativistic Doppler beaming of the jet radiation. Therefore, the extremely variable jet emission dominates the spectral energy distribution of blazars from the radio band up to the gamma rays. I will review the main results obtained through the analysis and interpretation of the multiwavelength variability of radio-loud AGN, focussing on blazars.

One of the main scenarios to account for the multi-wavelength flux variability observed in relativistic jets of active galactic nuclei (AGNs) is based on diffusive shock acceleration of a population of relativistic electrons on internal shocks of various origins. To understand the physical processes associated with the observed multi-wavelength emission maps and
light curves, we investigate the physics of the shocks in AGN jet. We simulate variable relativistic jets using the resolving the relativistic magneto-hydrodynamic simulation of fluid equation and the distribution of non-thermal electrons that are injected in shock regions. Synchrotron emission and radiative transfer are calculated in the post-processing for given observation angles and frequencies. With our scenario, we were able to explain the appearance of trailing components behind the leading injected variability. The latter destabilizes the jet, causing the emergence of oscillating standing shocks and relaxation shocks. Emissions from these regions can dominate the overall flux or lead to “flare echos” in the light curve.

I will show the results of the study of variability for a sample of more than 300 FSRQ in gamma-ray, focusing on waiting time between flares (L. Pacciani, A&A, 2022, 658, 164), and on flares luminosity and duration.
The investigation of waiting times revealed that gamma-ray activity can be modeled with overlapping bursts of flares, with flares uniformly distribuited within each burst, and bursts uniformly distribuited with a typical rate of 0.6/y.
Morerover, a statistically relevant fast component with timescale of order of days is revealed.
From this result, constraints on flares emission mechanisms were derived.
Timescales derived for FSRQ variability is very similar to the findings of Ivezic & MacLeod 2013, and of Burke 2021 for the damping timescale found in optical for SMBH of 10^8-10^9 solar masses.
Moreover, Kelly et al. (2009) aobserved that radio-loud quasars show an excess optical variability for timescales below 1 d, with a white noise PSD.
These similarities suggest a common origin for such a variability.

The characteristic variability of blazars is being since long time explained by relating it to a wide range of possible physical processes, occurring in the accretion disk and/or the jet. The various scenarios include emission spots in the accretion disk revolving around the supermassive black hole, magnetohydrodynamic instabilities in the disk or the jet, shocks traveling along turbulent jets, and relativistic effects due to the jet orientation. In the X-ray band, the background emission generated by the accretion disk seems to outshine any possible additional source of variability, such as the periodicity induced by the presence of a binary black hole in the central engine. The purpose of our work is the search of periodicity in the X-ray, UV and optical light curves of the blazar PG 1553+113 with Swift-XRT data spanning ten years from 2012 to 2022. This source is already known to exhibit periodic variability in the optical and the gamma-rays with a period of 2.2 yr only, we have performed a robust statistical analysis of the light curve. Our results confirm that the PG 1553+113 X-ray emission displays a periodicity shorter by a factor of ~40% than the gamma-ray one. We also investigated the cross-correlations between the light curves of this source in several bands, in search of possible time delays that could help to discriminate the spatial distribution of the various emitting region.

The supermassive black hole (SMBH) binary systems are important for testing the models of SMBH formation, comparing the physics of SMBH merging to gravitational wave (GW) detection, and determining the stochastic GW background at low frequencies, just to name a few.

We present an overview of current efforts on combining information from complementary techniques to detect close binary supermassive black holes (CB-SMBH, components bound in a Keplerian pair at mutual distances of less than 0.1 pc). This topic has typically been driven by theoretical work, but in recent years it has also generated interest in observational astronomy.

A variety of parameters influence CB-SMBH observability, the bulk of which are dictated by the system's evolutionary stage. Throughout electromagnetic domains, samples of dual SMBH systems separated by kiloparsecs to hundreds of parsecs have been detected. Nevertheless, the evidence is not as obvious at subparsec scales due to a lack of instrumental resolution to separate the binary components features in highly dimensional observability space.
Moreover, a binary SMBH ensemble should provide a stochastic gravitational wave (GW) background with a distinct strain form. The Pulsar Timing Array (PTA) might resolve massive or nearby SMBH binaries from the GW background, but only during the early inspiral stage of a binary merger, in addition to supplying GW information for the ensemble of binary systems.

Thus, to expand the explored parameter space of CB-SMBHs and follow them up with coming nano-Hz GW interferometers (or PTA), we should employ all available time domain observations to identify the physical parameters of CB-SMBHs and noteworthy candidates.
Using time domain data sets collected over several techniques, rather than just one, should enhance the amount and quality of information on the observed object. Even if one of the data sets is far more inaccurate than the other, this is supposed to be true.

Given the increasing amount of data from already existing and future large sky surveys, as well as the growing population of high resolution imaging tools capable of scanning individual objects with ever-sharper vision, the combined information from these advanced techniques has the potential to uncover a substantial portion of CB-SMBH candidates.

Supermassive black hole binaries lurk, often unseen, in the centers of post-merger galaxies, and numerous electromagnetic surveys are seeking evidence of these dynamic duos’ effects on their host galaxies. In this talk I’ll discuss our recent paper, which analyzed the capabilities of promising methods to search for electromagnetic signatures of supermassive black hole binaries in current and future time domain surveys, including the Catalina Real-Time Transient Survey (CRTS) and the upcoming Legacy Survey of Space and Time (LSST). In this paper, we used Bayesian methods to disentangle periodic SMBHB signals from intrinsic damped random walk variability in AGN light curves.
Through a careful analysis of parameter estimation and Bayesian model selection, we investigated the range of parameter space for which binary systems can be detected, and determined that the false-detection rate depends on the quality of the data and is minimal in LSST.
I’ll also discuss the promising implications this work has on the possibilities for multi-messenger astrophysics through partnerships with pulsar timing arrays, such as the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), which search for gravitational waves from these binaries.

Quasar variability is often modeled simplistically as originating from a point-like lamp post geometry with a damped random walk time dependence. We create more realistic simulations of variability propagation through quasar structure using a flexible and physically motivated quasar model that incorporates lensing by the SMBH, disk and broad-line reprocessing, and extended geometry of the variability source. Using this model, we derive realistic transfer functions and simulate LSST-like multi-band light curves for a wide range of quasar structure parameters and variability source geometries. We use these to explore the degree to which deviations from the simplistic lamp post models can be determined in upcoming time-domain surveys. We also analyze light curves from SDSS and other existing surveys to make preliminary constraints on the validity of these models.

Active galactic nuclei (AGN) are thought to be powered by the accretion of matter around supermassive black holes at the centers of galaxies. The time-dependent variability of an AGN's brightness can provide valuable insights into the physical characteristics of its underlying black hole. The variability can be well modeled by a damped random walk process described by a stochastic differential equation (SDE). Upcoming wide-field telescopes such as the Rubin Observatory Legacy Survey of Space and Time (LSST) are expected to observe 100 million AGN in multiple bandpass filters, so new methods need to be developed to analyze the large volume of light curve data. Latent SDEs are variational auto encoders (VAEs) with a neural SDE as the decoder. Latent SDEs are well suited for modeling the AGN time series, as they explicitly model the underlying dynamics. We modify latent SDEs to jointly reconstruct the unobserved portions of multivariate AGN light curves as well as infer their physical properties, such as the black hole mass. We train our model on a realistic physics-based simulation of ten-year LSST light curves and find our method outperforms a multi-output Gaussian process regression in light curve reconstruction. Our method has the potential to provide a deeper understanding of the physical properties of black holes and AGN variability and may be applicable to a wide range of other astronomical times series.

We present a new tool FANTASY (Fully Automated pythoN Tool for Agn Spectral analYsis) for multicomponent fitting of active galactic nuclei (AGN) spectra in the optical and near infrared wavelength band. Spectra are modeled by simultaneously fitting the underlying broken power-law continuum, predefined emission line (narrow, broad, coronal, etc.) lists, and an Fe II model, which is here extended to cover the wavelength range from 3700 to 11000A. The Fe II model, founded solely on atomic data, effectively describes the strong emission of the complex iron ion in the vicinity of the H$\gamma$ and H$\beta$ lines, but also near the H$\alpha$ line.
Here we present a case study of the application of FANTASY code on SDSS-RM spectra with S/N>20, with the aim to study the variability properties of Balmer lines, as well as of Fe II emission. One interesting finding is that when Fe II emission is present near Hbeta, it is also detected redward from H$\alpha$, potentially contaminating the broad H$\alpha$ line wings. We show that the FANTASY code is well optimised for bulk fitting of AGN type 1 spectra from SDSS, as it is flexible and easy to use, thus showing great potential for AGN spectral analysis in the coming spectral surveys.

The emerging all-sky multi-epoch surveys (e.g., ZTF, Rubin LSST) have started a new era of time-domain astronomy. The variable nature of AGN across all wavelengths presents us with unique opportunities to probe AGN physics via time-domain analysis. I will start this talk by reviewing the time-domain analysis techniques, traditional and machine-learning based, currently employed in AGN selection and characterization. Then, I will present our most recent work on modeling ~30,000 quasar UV/optical light curves as second-order continuous-time autoregressive moving-average (CARMA) processes and introduce the new software package--EzTao--that we developed to conduct the modeling task. Lastly, I will preview our ongoing work to improve the current CARMA modeling technique and provide an outlook for further developments that will maximize the science output of next-generation time-domain surveys like the Rubin LSST.

Brightness variations of active galactic nuclei (AGNs) provide an alternative way to identify AGN candidates that could be missed by more traditional selection techniques. In this talk, I will first present a new variability and color-based classifier, designed to identify multiple classes of transients, persistently variable, and non-variable sources, from the Zwicky Transient Facility (ZTF) Data Release 11 (DR11) and ZTF forced aperture photometry light curves of extended and point sources. The main motivation of this model is to identify AGN candidates, but it can be used for more general time-domain astronomy studies. We used a hierarchical local classifier per parent node approach, to classify a total of 17 classes, including non-variable objects, transients, and stochastic and periodic variables. With this model, we have been able to identify AGN candidates at different redshifts and with different ranges of mass and luminosity. Then, I will present an anomaly detection (AD) technique designed to identify AGN light curves with anomalous behaviors. The main aim of this work is to identify changing-state AGNs (CSAGNs) at different stages of the transition, but it can also be used for more general purposes. We modeled ZTF DR5 light curves of 230,458 AGNs with a Variational Recurrent Autoencoder (VRAE) architecture, that allowed us to obtain a set of attributes from the VRAE latent space that describes the general behavior of our sample. These attributes were then used as features for an Isolation Forest (IF) algorithm. We used the VRAE reconstruction errors and the IF anomaly score to select a sample of 8810 anomalies. Bogus candidates dominate these anomalies, but we were able to identify promising AGNs with anomalous variations.

Quasi-periodic eruptions (QPEs) are a novel phenomenon in high-energy astrophysics, and to date have only been confirmed to be observed in a small number of AGN. Characterised by high amplitude variability over relatively short timescales, QPEs have the potential to provide insights into the strong gravity regimes in the innermost regions of the accretion disks around AGN. To provide robust predictions of the physical mechanisms involved we need to find more QPE sources to broaden the understanding of the parameter space they inhabit. We use known observations of QPEs and simulated lightcurves to determine whether machine learning approaches can detect QPE sources, and then apply these trained networks to the latest release of the XMM Serendipitous Source Catalogue in the hunt for further candidates.

Active galactic nuclei (AGN) exhibit small amplitude, short timescale variability in their optical luminosities, of roughly a few tenths of a magnitude over periods of hours to years. But extreme variability of AGN - large luminosity changes that are a significant departure from the baseline variability - are known as AGN flares. These events are rare and their timescales poorly constrained, and most of the literature focuses on individual events. With surveys such as the Legacy Survey of Space and Time (LSST) promising millions of transient detections per night in the coming decade, there is a need for fast and efficient classification of AGN flares. The problem with the systematic detection of AGN flares is the ability to detect them against a variable baseline; the ability to define a signal as a significant departure from the ever-present variability is a statistical challenge. Recently, Gaussian Processes (GPs) have revolutionised the analysis of time-series data in many areas of astronomical research. However, they have seen limited uptake within AGN astronomy. Here we investigate the efficacy of Gaussian Processes to detect AGN flares in both simulated and real optical light curves. We show that a GP can successfully detect AGN flares with a false-positive rate of less than one per cent, and we present examples of AGN that show extreme variability.