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...
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,...
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...
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....
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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,...
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...
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...
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...
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...
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...
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...
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...
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 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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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)...
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.,...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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....
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...
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 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...
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...
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...
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...
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...
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)...
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...
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,...
Five QPE (quasi-periodic eruption) sources have been detected in the past few years. But so far, the mechanism of QPE is still unclear. In this talk, I will introduce you to a disk instability model based on \citet{2021ApJ...910...97P} (PLC21) to explain GSN 069 and other QPEs. We improve the work of PLC21 to include a non-zero viscous torque condition at the inner boundary of the disk and...
Extremely variable quasars (EVQs) are a population of sources showing large optical photometric variability revealed by time-domain surveys. The physical origin of such extreme variability is yet unclear. We construct the largest-ever sample of 14,012 EVQs using photometric data spanning over > 15 years from SDSS and Pan-STARRS1 and divided them into five sub-sample according to their...
The UV-optical continuum emission from accretion discs is known to vary on all timescales, from days to decades. Its statistical description as a random walk as well as deviations from that phenomenology have not given much insight into disc properties.
Here, we report our discovery of a universal variability structure function for quasars, which appears when the observer's clock ticks in...
The Atacama Cosmology Telescope (ACT) was a ground-based CMB experiment in the Atacama desert in Chile that observed the millimeter sky until 2022. Lightcurves have been obtained from flux measurements of point sources (AGN) in single-pass scans of ACT across the sky between 2013 and 2022. ACT currently has lightcurves for over 200 of its brightest point sources with measurements sampled on...
Compact Symmetric Objects (CSOs) are a class of compact jetted Active Galactic Nuclei (AGN) whose observed emission is not relativistically boosted towards us. While that makes them a unique class to understand jet processes, they are often misclassified and confused with other types of radio sources. Motivated by the need to clear the confusion in the literature, we compiled a catalog of 79...
Recent studies have reported on a possible evolution of the covering factor (CF) with redshift. The goal of the presentation is to answer the question if this evolution is real or whether selection effects play an important role. The presented analysis was based on cross-matched multiwavelength photometrical data from the five major surveys (SDSS, GALEX, UKIDSS, WISE, Spitzer). A sample of...
We report the discovery and NTT/EFOSC2 spectroscopic identification of a new bright doubly lensed quasar eRASS1 J050129.5-073309 at redshift $z=2.47\pm0.03$. The source was selected from the first all-sky survey of the Spectrum Roentgen Gamma (SRG) eROSITA telescope and the Gaia EDR3 catalog. eRASS1 J050129.5-073309 is the optically brightest object in our sample and possesses remarkable...
The reason for the missing red giants near the center of our Galaxy has long been debated. Over the past few decades, many publications and explanatory theories have been proposed for this phenomenon. A new analytical theory was suggested relatively recently, its essence is the idea of long-term ablation of the upper layers of the envelopes of red giants during repeated passages through a...
Narrow-line Seyfert 1 (NLS1) galaxies are a class of active galactic nuclei (AGN) identified almost 40 years ago, but still not well understood. They are preferentially hosted by disk-like galaxies, and harbour fast-growing, low-mass supermassive black holes, accreting at high Eddington ratios. Some tens of NLS1s have been detected in gamma-rays, proving the presence of powerful relativistic...
The disk-reverberation is a leading technique extensively used to study the size and structure of the accretion disk. However, the observed lag spectrum does not agree with the theoretical prediction. The observed time lags are often 2-3 times higher than the expected lags under the first-order reprocessing from the standard accretion disk. This discrepancy motivates us to explore the disk...
In this talk, I will present the results of a multi-epoch monitoring with NuSTAR and XMM-Newton of NGC 1358, a heavily obscured AGN whose properties made it an ideal changing look candidate.
The source was indeed found to be highly variable in line-of-sight column density (NHlos) over time-scales from weeks to years, even transitioning from a Compton thick state (NHlos>1E24 cm-2) to a Compton...
We show that, contrary to simple predictions, most AGNs show at best
only a small increase of lags inthe J, H, K, and L bands with
increasing wavelength. We suggest that a possible cause of this near
simultaneity of the variability from the near-IR to the mid-IR is that
the hot dust is in a hollow bi-conical outflow of which we only see
the near side. In the proposed model sublimation or...
AGN variability on minutes to hour-like timescales in the optical waveband is termed as intra-night optical variability (INOV). Such variations are used as an alternative tool to indirectly verify the presence of jets in AGNs. Here, we report the first attempt to systematically characterize INOV for a sample of radio-quiet narrow-line Seyfert1 galaxies (RQ-NLSy1s) that had shown multiple...
The Eddington ratio $\lambda_\mathrm{Edd}$\, is the key parameter that describes the accretion mode of active galactic nuclei (AGN). Among the different modes, high-$\lambda_\mathrm{Edd}\,$ accretion is particularly fascinating because of its implications in the context of accretion physics, as well as AGN feedback. However, due to their relative paucity in the local Universe (z$<$0.1), only a...
A few decades have passed since the identification of narrow-line Seyfert 1 (NLS1) galaxies as a subclass of active galactic nuclei (AGN). NLS1s show a Seyfert 1-like spectrum, but with emission line widths similar to those of Seyfert 2 spectra. Such features are often believed to be produced by a high accretion rate, close to the Eddington limit, coupled with a low-mass black hole ($<10^8...
The Narrow Line Seyfert 1 Mrk 335 has been observed in X-rays since 2000 and has shown to be highly and rapidly variable in flux and spectral shape, due to changes in the structure of the hot corona responsible for the primary X-ray emission via Comptonization. Its complex X-ray spectrum presents interesting features that need to be investigated in different states. While several studies have...
The High Altitude Water Cherenkov (HAWC) Observatory has been looking at the Northern sky in the TeV band since March 2015. Its long duty cycle (about 24 hours per day) allows an excellent continuous monitoring of the brightest Blazars and Radio Galaxies emitting in the gamma-ray regime. We present HAWC lights curves and TeV spectra of Mrk 421, Mrk 501 and M87 collected over 6 years.
The HAWC...
Here we study the variability of quasar light curves found in the LSST AGN Data Challenge (LSST_AGN_DC), a dataset compiled from various catalogs for testing key aspects of active galactic nuclei (AGN) science with the Rubin Observatory Legacy Survey of Space and Time (LSST). The distributions of quasar parameters in large databases may show bimodality or multimodality, thus as preprocessing...
Reverberation mapping is an effective technique to understand the structure and kinematics of broad-line region (BLR) as well as the mass of the black hole. It involves measuring the time delays between the variable continuum and emission line fluxes. The expected time delay varies with the wavelength as (wavelength)^(4/3). But the observations have shown that the measured time delays are...
In the present era of astronomical surveys, multi-wavelength studies are gaining ground. Active galactic nuclei (AGN), powered by the accretion of matter onto their central supermassive black hole, emit over a broad range of wavelengths from the X-ray to radio. Therefore, exploring AGN at multiple wavelengths is critical to understand the energetics of these powerful engines. Because AGN...
Several studies of actively accreting supermassive black holes have revealed that large amplitude variability often triggers significant spectral changes; a phenomenon known as changing look AGN (CLAGN). eROSITA through its successive all sky surveys, has made the detection of the sources using the X-ray band much more systematic. In 2020, eROSITA along with the Zwicky Transient Facility(ZTF)...
In the radio-quiet category of active galactic nuclei (AGN), the observed X-ray
emission is believed to originate in the hot corona situated close to the vicinity
of the accretion disk. Despite the numerous X-ray studies on AGN, we still do
not have a clear understanding of the nature of the corona, such as its geometry,
shape, location and the physical processes that power it. Parameters...
X-ray reverberation models provide an unprecedented view of accretion processes in active galactic nuclei (AGN), allowing us to probe deep into the innermost regions close to the singularity. To date, spectral and variability models have been successful in studying certain coronal structures and disc geometries, mandating increasingly specialized codes to simulate complex reverberating...
Reverberation mapping (RM) is the main tool by which spatially unresolved components of accreting black holes may be probed. To date, the size of the broad-line region in ~100 quasars has been successfully measured using RM. Here we outline the concept of photometric RM, implement the multivariate correlation-function formalism and present new results for a sample of around 40 quasars. We...
The quadruply lensed quasar Q2237+0305 at z = 1.695, known as the Einstein cross, has been known for years to be a privileged laboratory for microlensing studies due to very short time lags. The spectra of the image A reveals a strong magnification effect that distorts the broad CIV emission line, while the image D shows no microlensing induced variability. The BLR microlensing is...
While the unified model of quasar structure provides a concise description of diverse spectra, the physical origins of components (e.g., broad line region, dust torus, and narrow line region) are unresolved. To learn more about the structure of quasars, we have chosen to study Changing-State Quasars (also known as Changing-Look Quasars) as they offer the opportunity to observe structural...
Quasars (QSO) are variable sources in all wavelengths and in all time scales. Here we study the variability of the QSO accretion disk continuum emission using the new multi-epoch SDSS-V spectroscopic data in timescales of days to months. We use a spectral decomposition method to measure the disk emission and the high cadence spectral data to characterize the disk variability as a function of a...
X-ray Quasi-Periodic Eruptions (QPEs) are a novel X-ray variability phenomenon associated with supermassive black holes. QPEs are short-lived, high-amplitude, soft X-ray bursts typically recurring every few hours over an otherwise stable quiescent level. QPEs were first observed in the (repeating) TDE candidate GSN 069 by XMM-Newton (2019), and they have now been detected in the nuclei of...
Photoionised gas at all scales is ubiquitously observed in AGNs, from
the optical up to the X-rays. Its density, geometry, velocity represent
a unique probe of the innermost accretion disc-scale, as well as on the
feeding and feedback connecting the AGN to the host environment.
However, current photoionisation codes usually assume time-equilibrium
and, thus, cannot self-consistently model...
The variable UV/optical emission results from the accretion disk reprocessing of the highly fluctuating X-ray emission. This can be tested by measuring inter-band time lags of quasars with different X-ray power. We report the inter-band time lag in an X-ray weak quasar, SDSS J153913.47+395423.4. We found a significant cross-correlation with a time delay of 32 days (observed-frame) detected in...
The near X-ray bright Seyfert 2 galaxy NGC 2992 was extensively observed by XMM-Newton, NuSTAR and Swift from 2019 to the end of 2021. The resulting exposures provide a compelling multi-epoch dataset to test for the properties of this source across different timescales, from hours up to years. Our analysis revealed the X-ray emission of NGC 2992 to show remarkable changes (larger than a factor...
The variability of active galactic nuclei (AGN) across all wavelength bands is considered to be one of their most defining characteristics. Generally, their variability is assumed to be of stochastic nature and has been used with great success in the last 30 years to identify and map the innermost AGN structures -- namely the broad-line region (BLR) and accretion disk (AD) -- using methods...
Thanks to the advent of large-scale optical surveys, a diverse set of flares from the nuclear regions of galaxies has recently been discovered. These include the disruption of stars by supermassive black holes at the centres of galaxies - nuclear transients known as tidal disruption events (TDEs). Active galactic nuclei (AGN) can show extreme changes in the brightness and emission line...
Recent intensive reverberation mapping campaigns of AGN are opening a new window in the studies of the accretion geometry around super massive black holes. Here we present the X- ray/UV/optical lag spectrum of the high accretion rate AGN Mrk 110 during three epochs between 2017 and 2019. We monitored the source using Swift, Las Cumbres Observatory and the Zowada Observatory. During the first...
Active galactic nuclei (AGN) are powered by accreting supermassive black holes, surrounded by a torus of obscuring material. The exact geometry of this material has been a subject of debate, as models have advanced from the initial homogeneous torus to a variety of possibilities, ranging from cloud distributions, to warped disks, to outflows. Recent studies have shown how the torus structure,...
Accretion onto black holes and other compact objects occurs across a wide range of scales. Despite the diversity in the physics involved, the variability shows a remarkable similarity in its properties. The theory of propagating fluctuations, in which random fluctuations within an accretion disk travel inwards and combine, has long been used to explain this variability. Recent numerical work...
Current wide-field quasar surveys are not sensitive to low-luminosity active galactic nuclei (AGN), due to both flux limit and color selection effects. The absence of such objects will bias our understanding of AGN evolution, and limit the ability to distinguish between various models of the origin and seed evolution of supermassive black holes (SMBHs). Variability is an ubiquitous signature...
Binary and dual active galactic nuclei (AGN) are an important observational tool for studying the dynamical evolution of galaxies and supermassive black holes (SMBH). However, they are notoriously difficult to unambiguously detect due to current observational limits and biases, and are often identified serendipitously. An entirely new method for identifying possible AGN pairs makes use of the...
Obscuration in active galactic nuclei (AGN) has been largely studied all over the electromagnetic spectrum. It is commonly accepted that the obscuration is caused by a “dusty torus”, i.e., a distribution of molecular gas and dust located at ∼1–10 pc from the accreting supermassive black hole (SMBH). While the existence of this obscuring material is universally accepted, its geometry and...
