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...
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...
