The Galactic Center harbors the nearest massive black hole. With a distance of only 8 kpc, it is the closest laboratory to study the astrophysical processes at work in these extreme objects, and to probe Einstein's general theory of relativity in the regime of strong gravity. Our presentation gives an overview of adaptive optics and GRAVITY infrared interferometry observations of stellar...
The Einstein Probe (EP) is a space X-ray observatory designed to detect mainly high-energy transient and variable sources in the universe. It features a lobster-eye wide-field X-ray monitor with unprecedented sensitivity and spatial resolution in the soft X-ray band, and a conventional X-ray telescope to perform quick onboard follow-up observations. The Einstein Probe is a project led by the...
The Einstein Probe (EP) is a newly launched space mission dedicated to the X-ray all-sky survey and characterization of all kinds of high-energy transients. During the first few months of in-orbit operation, EP has discovered dozens of new X-ray transients with intriguing properties. In this talk, I will present a few typical examples of TDE candidates discovered by EP since its launch,...
I will present our new work on modeling the 15-parsec-sized Sgr A lobes observed at the Galactic center as the relic of a tidal disruption event occurring about 3500 years ago by the supermassive black hole Sgr A* in our Galaxy. While an increasing number of extragalactic TDEs have been identified in recent years, TDE flares have never been detected from the Galactic center. A TDE flare...
In TDEs, the stream-stream collision is an important way to dissipate orbital energy in the process of circularizing the debris stream. For non-rotating black holes (BHs), due to the apsidal precession the stream intersects with itself near the its orbital apocenter after a single winding. However, for rotating BHs, the out-of-plane precession arising from the Lense-Thirring effect may prevent...
Quasi-periodic Eruptions (QPEs) are luminous bursts of soft X-rays from the nuclei of galaxies, repeating on timescales of hours to weeks. The mechanism behind these rare systems is uncertain, but most theories involve accretion disks around supermassive black holes (SMBHs), undergoing instabilities or interacting with a stellar object in a close orbit. It has been suggested that this disk...
The discovery of tidal disruption events (TDEs) has experienced great progresses in the past decade thanks to modern wide-field time-domain surveys. I will first review the current search of TDEs in the optical band. Then I will present our systematic search for TDEs using the novel dust infrared echoes (MIRONG project), which have revealed a large sample of dusty TDE candidates missed by...
ASASSN-14ko is a nuclear transient resulting from the repeated partial tidal disruption event (rpTDE). We conducted high-cadence, multiwavelength observations of this source, revealing several recurrent early bumps and rebrightenings in its UV/optical light curves. The energy released during these bumps and rebrightenings shows a decreasing trend in recent UV/optical outbursts and we monitored...
Partial TDE (pTDE) complements the classical full TDE (fTDE) picture, predicted to have higher occurrence rate than fTDE. Although it is still hard to distinguish a pTDE from a fTDE through a single flare, a repeated pTDE can provide multiple flares for a robust identification. In this talk, I will introduce the discovery and follow-up observations of a highly-confident repeated pTDE, AT...
AT 2022dbl was a typical optical-UV tidal disruption event (TDE) in a typical TDE host galaxy. Then, 700 days after the first flare, a second, almost identical, flare occurred at the same position. Ruling out lensing and two unrelated events, we conclude that the first flare of AT 2022dbl was a partial disruption. The question remains whether the second flare was a full disruption or another...
Quasi-periodic eruptions (QPEs) are an emerging class of high amplitude bursts of X-ray radiation, repeating on a hours-day timescale, recently discovered near the central supermassive black holes (SMBHs) of a few low-mass galaxies. I will briefly review our current theoretical understanding of QPEs, and the different classes of proposed theoretical interpretations. I will focus on a scenario...
Quasi-periodic eruptions (QPEs) are intense repeating soft X-ray bursts with recurrence times about a few hours to a few weeks from galactic nuclei. Though the debates on the origin of QPEs have not completely settled down, more and more analyses favor the interpretation that QPEs are the result of collisions between a stellar mass object (a stellar mass black hole or a main sequence star) and...
Quasi-periodic eruptions (QPEs) are intense repeating soft X-ray bursts with recurrence times about a few hours to a few weeks from galactic nuclei. Though the origin of QPEs is still an open question, more and more analyses favor the interpretation that QPEs are the result of collisions between a stellar mass object (SMO, a stellar mass black hole or a main sequence star) and an accretion...
Within the past decade, it has become clear that tidal disruption events (TDEs) can power a variety of jets and outflows, cementing their status as premier probes of accretion processes in supermassive black holes (SMBHs). Radio observations have been central to characterizing the properties of out- flowing material from TDEs, as they reveal synchrotron radiation produced in the shock formed...
Recent radio observations discovered that a significant fraction of TDEs are accompanied by late-time radio flare ~1000 days after the discovery. One possible origin of the flares is a relativistic jet. By applying a generalized formalizm to analyze radio emissions, I will discuss the possibility that if viewed from off-axis, a powerful jet (as observed in AT2022cmc) can power the late time...
The CLAGN 1ES 1927+654 was known as a type 2 Seyfert galaxy, which exhibited drastic variability recently in ultraviolet (UV)/optical and X-ray bands. An UV/optical outburst was observed in the end of 2017, and it reached the peak luminosity ~50 d later. The high-cadence observations showed a rapid X-ray flux decline with complete disappearance of the power-law hard X-ray component when the...
Tidal Disruption Events (TDEs) in Active Galactic Nuclei (AGN) exhibit distinct observational signatures compared to those in quiescent galaxies, attributed to the denser disruption environments within the accretion disk. Additionally, detecting these events poses challenges due to the luminous background emissions from AGNs. Consequently, TDEs in AGNs have been historically overlooked, with...
It is still in dispute the existence of intermediate-mass black holes (IMBHs) with a mass of ~10^3-10^5 solar masses (Msun) which are the missing link between stellar-mass black holes (5-50 Msun) and supermassive black holes (10^6-10^10Msun). Many candidates have been proposed including the black holes in dwarf galaxies, globular clusters, and hyperluminous off-nuclear X-ray sources. Until...
In recent years, a class of tidal disruption events (TDEs) with special light curves has been observed: during the luminosity decline phase, they will brighten again and produce a second peak. The interval between the two peaks is only a few hundred days, and the flux of the second peak may become higher or lower than the first peak. There are many models for this phenomenon in the literature,...
Many stars and stellar compact objects exist in the nuclear region of galaxies. The stellar dynamics and related TDEs/EMRIs are strongly affected by the central SMBH activities (or appearance or disappearance of accretion disk). I will present the evolution of TDE rate considering the evolution from quiescent to active galaxies, which can roughly explain why the TDEs are preferred to be found...
Recently, multiple detections of repeating soft X-ray emissions from quiescent galaxies have suggested a potential new category of nuclear transient events: quasi-periodic eruptions (QPEs). These events are characterized by quasi-regular, large-amplitude X-ray flares detected at around ~100 eV, with typical recurrence times ranging from hours to days, a connection to previous tidal disruption...
We present a noteworthy transient AT 2021aeuk exhibiting three distinct optical flares between 2018 and 2023. It is hosted in a narrow-line Seyfert 1 galaxy. One of the flares exhibits rapid rises, and long-term decays over 1000 days. We applied quantitative analysis with light curve fitting and blackbody fitting, which cannot confidently clarify the origin of triple flares. The flares could...
In galactic centers, stars and binaries can be injected into low-angular-momentum orbits, resulting in close encounters with the central supermassive black hole (SMBH). We use simulations to study such encounters under a wide range of conditions. Depending on the system parameters (such as $\beta_b$, the ratio of binary tidal radius to pericenter distance $r_p$ to the SMBH, and the...
Active galactic nuclei (AGN) can exhibit drastic changes that differ significantly from the normal stochastic variability, such as changing-look AGN and rapid-turn on events. Transients also occur in quiescent supermassive black hole (SMBH). When it tidally disrupts a passing star, about half of the debris can be accreted to produce a luminous flare in a tidal disruption event (TDE). It is...
The tidal disruption event(TDE) in GSN 069 is associated with X-ray quasi-periodic eruptions(QPEs) and re-brightened about 10 years after the first flare. The QPEs is probably produced by the mass transfer of a star in a high eccentric orbit around a central supermassive black hole(SMBH), which is captured by SMBH from a binary by the Hills mechanism. In this scenario, the TDE cannot be...
Eintein Probe (EP) mission is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability, and was lauched on January 9, 2024. It carried out a series of performance verification and calibration observation after launch and had begun scienfic observations since late July. Its main scientific objectives include monitoring of the known X-ray sources, like XRBs, AGNs,...
Changing-look active galactic nuclei (AGNs), which transition between quasar-like and quiescent states, have become a critical focus in astrophysical research. The Dark Energy Spectroscopic Instrument (DESI) offers an unprecedented opportunity to detect and study these phenomena on a large scale. Utilizing DESI's high-precision spectroscopic data, we have recently identified and analyzed...
Accreting black holes are highly dynamic systems, where particles are efficiently accelerated during the complex inflow/outflow processes, producing multi-messenger emissions. In this talk, I will focus on particle acceleration in jets and their corresponding multi-wavelength emissions, while also touching on the potential roles of inner accretion flows and the possible origin of neutrinos....
Mergers of black hole (BH) binaries embedded in the active galactic nucleus (AGN) accretion disks have recently received significant attention as an interesting type of gravitational wave (GW) source. Studying the formation processes of these binaries allows us to better understand the rate and the observational properties of their mergers. In this talk, I will present our work on the...
In this talk, I will discuss the unprecedented science opportunities to probe astrophysical environments of galactic-centre massive black holes by observing extreme mass-ratio insprirals with gravitational waves and electromagnetic counterparts. Possible measurements target both direct modification of the gravitational waveform due to the environment and indirect change of system parameters...
Horizon-scale images of black holes (BHs) and their shadows have opened an unprecedented window onto tests of gravity and fundamental physics in the strong-field regime. I discuss the prospects for witnessing the presence of new physics out BH imaging, using an array of observables such as the time evolution of the shadow and the photon ring autocorrelation. I focus on the properties of new...
In the disks of active galactic nuclei (AGN), compact objects and stars are thought to be embedded and densely populated. These compact objects are predicted to merge with each other, representing one of the promising channels for stellar-mass black hole (BH) mergers discovered through gravitational waves (GWs). Due to the uncertainties of these processes, it is of great interest to identify...
Stellar black holes (sBHs) are widely believed to exist in the accretion disks of active galactic nuclei (AGNs). Previous studies often focus on the transient emission produced by embedded sBHs. Here, we explore the possible observational consequences of an AGN accretion disk that contains a population of accreting sBHs. Embedded accreting sBHs change the effective temperature distribution of...
Circumbinary disks are expected to exist around supermassive
black-hole binaries as a consequence of accretion from the
interstellar medium following galaxy mergers. I will discuss recent
works on numerical modeling of circumbinary accretion, focusing
short-term and long-term variabilities, and angular momentum transfer
between the disk and the binary -- the result suggests that...
We study the dynamics of a star orbiting a merging black-hole binary (BHB) in a coplanar triple configuration. During the BHB’s orbital decay, the system can be driven across the apsidal precession resonance, where the apsidal precession rate of the stellar orbit matches that of the inner BHB. As a result, the system gets captured into a state of resonance advection until the merger of the...
The inner structures of active galactic nuclei (AGNs) are essential for probing the fundamental parameters of the central supermassive black hole (BH), understanding accretion physics, and investigating the physical connection between the BH and its host galaxy. The reverberation mapping (RM) technique has been widely used to study the broad-line region (BLR), the dust torus, the accretion...
How to seperate the variability by TDE from others is important? We need to understand the variability in the nuclie better. We investigate the underlying stochastic processes driving the multi-wavelength variability of the blazar BL Lacertae over the past two decades by comprehensive multiband data set including Fermi-LAT (γ-ray), Swift-XRT (X-ray), Swift-UVOT (ultraviolet), and the MOJAVE...
At high accretion rates, the high temperature and low density in the standard thin disk (SSD) suppress the absorption of the radiation, making the innermost region effectively optically thin. In this work, we systematically investigate the effectively optically thin accretion flow through the generalized self-similar solution of the steady axisymmetric, non-relativistic accretion flow. Our...
With newer and more precise observations of super-Eddington systems, it has become increasingly important to model the spectral emissions from slim accretion disks. However, X-ray reflection from such sources has not been thoroughly explored. REFLUX is a newly developed relativistic disk reflection model specifically tailored for super-Eddington systems. The model incorporates a slim disk...
Recent gravitational wave observations have detected several tens of mergers between stellar mass and likely intermediate-mass black holes (BHs). However, no clear observational evidence has yet been found for electromagnetic (EM) signals associated with such BH mergers or binary BHs. If a dense gas surrounds binary BHs, a circumbinary disk (CBD) can be formed. In such a case, the system...
Sagittarius A (Sgr A), the supermassive black hole in the Galactic Center is of significant interest to the astrophysical community as a natural laboratory due to its proximity and relatively high angular resolution in the sky. This is true now more than ever given the exciting recent results from the Event Horizon Telescope and the GRAVITY interferometer. Our understanding of Sgr A* has...
Sgr A∗ exhibits flares at various wavelengths, but their origin remains unclear. Magnetic flux rope eruption from the black hole is one of the possible candidates for explaining the observed Sgr A* flares. Based on new 3D GRMHD simulations of magnetized accretion flows, we investigate the dynamics of magnetic flux ropes ejected from the vicinity of the black hole. We found the formation of...
The UV/optical light curves observed in active galactic nuclei (AGNs) are well-characterized by damped random walk (DRW) processes, where the damping timescale $\tau_\text{d}$ shows correlations with both black hole mass ($M_\text{BH}$) and photon wavelength ($\lambda$). However, the underlying physical mechanisms behind these scaling laws remain uncertain. We aim to understand the variability...
Massive stars formed in the self-gravitating regions of accretion disks remain stellar-mass black holes in active galactic nuclei forming a special population of black holes as satellites. In this presentation, I would talk about the observational consequences of these black holes.
SgrA* often shows bright, episodic flares observationally, the mechanism of the flares intermittent brightening is not very clear. Many people believe the flares may formed by the non-thermal particles, which can be a consequence of the magnetic reconnection and shock waves. In this work, we use MHD simulation to study the magnetic reconnection process which is on the accretion disk, and show...
Accretion disks around black holes are typically thought to be misaligned with the black hole’s spin axis due to the random angular momentum of infalling gas. Using 3D general relativistic magnetohydrodynamic (GRMHD) simulations, we investigate the accretion process of tilted disks. Contrary to the conventional understanding that prograde precession is driven by the Lense-Thirring effect, we...
The formation of jets in black hole accretion systems is a long-standing problem. It has been proposed that a jet can be formed by extracting the rotation energy of the black hole (“BZ-jet”) or the accretion flow (“disk-jet”). While both models can produce collimated relativistic outflows, neither has successfully explained the observed jet morphology. By using general relativistic...
There are ample ambiguities in the theoretical modeling of accretion flow around Sgr A*.The theory and observations can not confirm the nature of the accretion flow around the central supermassive black hole. In this talk, we investigate the possible application of low-angular momentum flow for the same. We focus on the role of angular momentum in determining the properties of accretion flows...
Thanks to modern time-domain surveys, the process of identifying a tidal disruption event (TDE) gradually develops in the quiescent galaxy and several repeating partial TDEs have been studied. Meanwhile, the TDE in AGN is theoretically predicted to exhibit different observable features from the identified TDEs in quiescent galaxies. To date, a dozen of TDE in AGN candidates and the ambiguous...
The tidal disruption of a star by a nearby black hole can trigger transient electromagnetic signals in multiple wavebands, making it a well-recognized class of transient events. Understanding the origin of detected emissions from such tidal disruption events (TDEs) is particularly interesting.
In this work, we discuss the potential radiative emissions from TDEs during the formation of the...
Supersoft hyperluminous X-ray sources offer a unique opportunity to study accretion onto supermassive black holes in extreme conditions. Typical supersoft hyperluminous sources are tidal disruption events, quasi-periodic eruptions, changing-look AGN, and other anomalous nuclear transients.
Although these objects are rare phenomena amongst the population of X-ray sources, we developed an...
The orbital evolution of binary black hole (BBH) systems is determined by the component masses and spins of the black holes and the governing gravity theory. General relativity (GR) is the simplest theory of gravity that lays the foundation for successfully explaining the current gravitational wave (GW) observations. We present a method of **stacking up the time-frequency pixel energies...
