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...
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...
