Extreme AGNs and Transients: When Black Holes Misbehave

4 Aug 2025, 09:00 → 8 Aug 2025, 13:00 Asia/Hong_Kong

The University of Hong Kong

Jane Dai (lixindai@hku.hk)

Overview

Recent discoveries have revealed supermassive black holes behaving in extraordinary ways - and these emerging fields are now among the hottest topics in astrophysics.

This invitation-only workshop focuses on the most dynamic and enigmatic supermassive black hole phenomena—violent TDEs, rapidly transitioning CL-AGNs, and the mysterious LRDs—where extreme physics challenges our fundamental understanding. Fueled by revolutionary data from Einstein Probe, GRAVITY+, and JWST, these emerging fields are reshaping black hole science and galaxy evolution.

Join leading theorists and observers to explore the latest developments, open questions, and future directions in these rapidly evolving frontiers.

Themes

1. Tidal Disruption Events (TDEs) 
2. Changing-Look Active Galactic Nuclei (CL-AGNs)
3. Little Red Dots (LRDs)
4. Advanced telescopes: Einstein Probe, GRAVITY+, JWST, etc 

Senior Participants

Lixin Dai (HKU), Luis Ho (KIAA), Chichuan Jin (NAOC),  Claudio Ricci (Geneva), Jinyi Shangguan (KIAA), Benny Trakhtenbrot (Tel Aviv), Qingwen Wu (HUST)

Important Dates

Event	Date
Abstract Submission Opens	8 July, 2025
Abstract/Registration Deadline	18 July, 2025
Registration Fee Deadline	2 August, 2025
Workshop Dates	4 - 8 August, 2025

 

Registration Fee

Professor: HKD 1,600

Student, Post-doc: HKD 800

 

Scientific Organization Committee (SOC)

Jane Dai (HKU) & Luis Ho (KIAA)

Local Organization Committee (LOC)

Jane Dai (HKU), Rudrani Kar Chowdhury (HKU), Zoe Kan (HKU), Muses Wong (HKU)

Acknowledgement

We acknowledge the kind support from the Physics Department of the University of Hong Kong and Hong Kong Institute of Quantum Science & Technology.

 

campus_map.pdf

Directions.pdf

Monday 4 August

09:30 → 10:15 Registration

10:15 → 10:30 Opening Remarks

10:30 → 12:00 LRDs: Luis Ho

10:30 Little Red Dots: Laboratory for Black Hole and Galaxy Formation

The discovery of the "little red dots" with the James Webb Space Telescope has revealed a host of surprises about this mysterious class of high-redshift objects. Their physical nature remains hotly contested. I will argue that little red dots trace the initial phase of supermassive black holes that predates the formation of their host galaxies. As such, they offer a uniquely valuable laboratory to illuminate the physical mechanism of black hole formation and the initial conditions of the earliest moments of galaxy formation. I will outline what we know and the much else that still remains uncertain.

Speaker: Prof. Luis Ho (Kavli Institute for Astronomy and Astrophysics, Peking University)

Luis_Ho.pdf

12:00 → 13:50 Lunch

13:50 → 15:00 LRDs: Chenxuan Zhang & Chang-Hao Chen

13:50 Double Disk Model for Little Red Dots: an Intrinsic Luminous Infrared Model

One of the most mysterious results from observations of the James Webb Space Telescope (JWST) is the detection of numerous, high-redshift, very red, extremely compact, broad-line sources termed ``little red dots'' (LRDs). It is unclear whether the LRDs belong to an active galactic nucleus (AGN) or simply a collection of very compact star clusters. We build spectral energy distributions (SEDs) for 29 LRDs at $z \approx 3-8.5$ based on JWST photometric and spectroscopic observations. We find that the V-shaped SEDs of these LRDs exhibit a roughly similar break frequency at $\nu_{\rm b}=10^{14.96\pm0.06}$ Hz, which corresponds to $\lambda_{\rm b}=3287_{-424}^{+487} \text{\AA}$ in the rest frame. We propose that this unique SED can be explained by the combination of an inner standard disk and an outer gravitationally unstable accretion disk with Toomre parameter $Q\sim1$. The outer disk has a temperature of $\sim2000-4000$ K for typical AGN parameters, which can well reproduce the near-infrared to optical bump as observed in LRDs. This model can naturally explain the strong infrared to optical emission and the V-shaped SED with a similar break frequency $\simeq 10^{15}$ Hz for LRDs without invoking strong dust extinction or unusual stellar contribution from a host galaxy. Most LRDs stay in sub-Eddington state based on the SED modeling, which are intrinsically weak in X-rays.

Speaker: Mr Chenxuan Zhang

Chenxuan_Zhang.pdf

14:25 The Host Galaxy And Complex UV Morphology of the Little Red Dots

We investigate the morphology from UV to optical of eight little red dots (LRDs) selected from the JWST UNCOVER survey, applying a new technique (GalfitS) to simultaneously fit the morphology and spectra energy distribution using multiband NIRCam images covering ~ 1-4 ${\mu m}$. For the majority of LRDs (7/8) in our sample, the morphology in all bands are dominated by an unresolved point source, with no concentric host galaxy component detected. We detect the host galaxy in only one LRD, MSAID38108 at z=4.96, which has a stellar mass of ${{\rm log}\ (M_*/M_{\odot})=8.66}$, an effective radius of 0.66 kpc, and a Sersic index of n=0.71. Based on the black hole masses estimated from the broad Ha emission line, the derived stellar mass limits are at least a factor of 10 lower than expected from the ${z\approx 0}$ scaling relation. Intriguingly, four of the LRDs (50% of the sample) show extended, off-centered emission that is particularly prominent in the bluer bands. The SED shape of the off-centered emission suggests a combination of foreground low-redshift interlopers, companion galaxies, and/or nebular emission originating from low-density, low-metallicity gas ionized by the LRDs.

Speaker: Mr Chang-Hao Chen (The Kavli Institute for Astronomy and Astrophysics at Peking University)

Changhao_Chen.pdf

15:00 → 15:30 AGNs: Ruancun Li

15:00 The Dichotomy in the Nuclear and Host Galaxy Properties of High-redshift Quasars

The early growth of high-redshift quasars and their host galaxies raises critical questions about their cosmic evolution. We exploit the angular resolution and sensitivity of NIRCam to investigate the host galaxies of 31 quasars at $4\lesssim z\lesssim7$ drawn from multiple JWST surveys. Using a new multi-band forward-modeling code (\textsc{GalfitS}) that incorporates physically motivated priors, we securely detect and quantify the host emission in 30 objects, while simultaneously characterizing the nuclear spectral energy distribution. The host galaxies of high-redshift quasars are $\sim 0.3$~dex more compact than star-forming galaxies of comparable mass. A striking dichotomy emerges: luminous "blue'' quasars ($L_{5100}\gtrsim10^{45}\,{\rm erg\,s^{-1}}$) reside in bulge-dominated galaxies ($n \approx 5$) and exhibit a narrow range of ultraviolet nuclear slopes (median $\beta_{\rm UV} \approx -1.4$), while fainter ``red'' quasars inhabit disk-like hosts ($n\approx 1$) and display a broad range of slopes ($\beta_{\rm UV}\approx-2$ to 4). These two populations differ markedly in their black hole-to-stellar mass ratios, with high-luminosity quasars showing $M_{\mathrm{BH}}/M_\ast = 1.2\%$ compared to $4.7\%$ for lower luminosity sources, placing them collectively $\sim$0.6~dex above the local $M_{\mathrm{BH}}-M_\ast$ relation. This offset likely reflects rapid black hole growth in early gas-rich environments, where feedback from the active galactic nucleus becomes effective only after substantial gas depletion. Our findings suggest that the observed dichotomy, whether due to intrinsic spectral differences or dust extinction, fundamentally shapes the coevolution of supermassive black holes and their host galaxies in the early Universe.

Speaker: Dr Ruancun Li (Peking University)

Ruancun_Li.pdf

15:30 → 16:20 Discussion Time

Tuesday 5 August

13:00 → 15:00 TDEs: Lixin Dai & Chichuan Jin

13:00 Tidal Disruption Events: Extreme Transients Probing Massive Black Holes

Speaker: Prof. Jane Lixin Dai (The University of Hong Kong)

Jane_Lixin_Dai.pdf

14:00 Early Discoveries of TDE Candidates by the Einstein Probe Mission

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, including the first case of TDE spectroscopically confirmed to be located in the outskirts of a galaxy. Its rising, peak, plateau and decay phases have all been observed, supporting it in hosting an IMBH of tens of thousands of solar masses. Then I will introduce some puzzling TDE candidates discovered by EP, which are still being monitored and under investigation. Finally, I will provide some prospects for future discoveries and joint observations of EP with other facilities on TDE sciences.

Speaker: Dr Chichuan Jin (National Astronomical Observatories, Chinese Academy of Sciences)

15:00 → 15:20 Break

15:20 → 17:20 TDEs: Rudrani Kar Chowdhury, Jin-Hong Chen, Mengye Wang, Janet Chang & Xiangli Lei

15:20 TDEs on FIRE: Co-Evolution of Tidal Disruption Event Rates and Host Galaxies across Cosmic Time

Tidal disruption events (TDEs) have been thoroughly examined within the local universe using diverse observations and theoretical frameworks. However, the study of TDEs at high redshifts has been relatively scarce. The imminent launch of advanced high-resolution, wide-field surveys facilitated by telescopes such as LSST, Nancy Grace Roman Space Telescope (Roman) and Ultrasat is set to revolutionize our understanding of TDEs in the early universe. For the first time, we have explored the rates of TDEs and their correlation with various host galaxy properties across a wide redshift range (z=1-10) using the state-of-the-art cosmological zoom-in simulation FIRE-2. Unprecedented resolution of FIRE-2 provides realistic stellar density profiles at the galaxy centres, enabling accurate calculations of tidal disruption rates (TDR) using loss-cone theory. In this talk, I will discuss the key findings from our study, including the strong correlation of TDR with black hole and host galaxy mass, the connection between TDR and cosmic star formation history, and the potential for observing off-nuclear TDEs with LSST, Roman, and Ultrasat at different redshifts.

Speaker: Dr Rudrani Kar Chowdhury (The University of Hong Kong)

Rudrani_Kar_Chowdhury.pdf

15:50 Precessing Super-Eddington Disks in Tidal Disruption Events: Dynamics and Periodic Multiwavelength Variability

A tidal disruption event (TDE) occurs when a star ventures too close to a massive black hole. Initially, the angular momentum of the star's orbit may not coincide with the black hole's spin axis, resulting in the formation of a tilted accretion disk relative to the black hole. The Lense-Thirring effect then sets this disk into precession. In this study, we have constructed a precessing super-Eddington TDE disk model, drawing on the outcomes of simulations. Using this model, we calculate the spectra from a range of viewing angles. As the disk precesses, the viewing angle changes over time, leading to a periodic evolution of the light curve. Our analysis demonstrates that the X-ray flux is highly variable, responding to the shifts in viewing angle caused by disk precession. In contrast, the optical and near-ultraviolet (NUV) emissions, which originate from the reprocessed material in the outflow, exhibit relatively less variability. We have identified three distinct regimes of variability driven by disk precession. Firstly, when the precession occurs within a narrow range of viewing angles, the light curve experiences only minimal modulations. Secondly, within a moderate range of viewing angles, intense X-ray variability is observed. This includes instances where X-rays disappear at viewing angles $\gtrsim 50^{\circ}$, and there is an inverse correlation between X-ray and optical/NUV emissions. Thirdly, when the precession spans an extremely wide range of viewing angles, substantial modulations are seen across both the X-ray and optical/NUV bands. For precession periods longer than one year, there is a possibility of late - time rapid X-ray brightening. This phenomenon could potentially account for the late-time X-ray activity observed in some TDEs. Our model offers a more profound understanding of the dynamics of precessing accretion disks and the underlying physics of TDEs.

Speaker: Dr Jin Hong Chen (Sun Yat-Sen University)

Jin-Hong_Chen.pdf

16:20 An Explanation for the Overrepresentation of Tidal Disruption Events in Post-starburst Galaxies

TDEs provide a valuable probe for studying the dynamics of stars in the nuclear environments of galaxies. Recent observations show that TDEs are strongly overrepresented in post-starburst or “green valley” galaxies, although the underlying physical mechanism remains unclear. Considering that galaxy mergers may trigger starbursts and AGN activity, the TDE rate may also be related to the evolutionary stage following the merger. In this work, we revisit TDE rates by incorporating an evolving AGN disk within the framework of “loss cone” theory. We numerically evolve the Fokker–Planck equations by considering star–disk interactions, in situ star formation in the unstable region of the outer AGN disk, and evolution of the accretion process of supermassive black holes. We find that the TDE rates are enhanced by about 2 orders of magnitude shortly after the AGN transitions into an inactive stage. During this phase, the accumulated stars rapidly scatter into the loss cone due to the disappearance of the inner standard thin disk. Our results provide an explanation for the overrepresentation of TDEs in post-starburst galaxies.

Speaker: Mr Mengye Wang (Department of Astronomy, School of physics, Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan, China)

Mengye_Wang.pdf

16:40 A New Study on the Morphology of TDE Host Galaxies

Tidal Disruption Events (TDEs) host galaxies are known to exhibit distinct characteristics, such as high central stellar concentration and an overrepresentation in E+A (post-starburst) galaxies, which has led to the hypothesis that recent merger events may play a significant role in their evolution. However, the underlying mechanisms driving these characteristics remain unclear. This study investigates TDE host galaxies using data from the Sloan Digital Sky Survey (SDSS) catalog and high-resolution images from the Canada-France Imaging Survey, focusing on their morphological properties and merger indicators. By comparing TDE host galaxies with carefully matched control galaxies, we confirm their elevated central concentration but find no evidence of enhanced post-merger activity, as inferred from both their image morphology and convolutional neural network predictions of post-merger features. Instead, we identify a significantly higher fraction of barred and ring structures in TDE host galaxies, particularly within the green valley population. These findings challenge the conventional assumption that recent mergers are the primary driver of central stellar density buildup in TDE host galaxies. Hence, we propose that the presence of bars may serve as an alternative mechanism, facilitating the transport of stellar material to the galactic center and enhancing the central concentration for TDE host galaxies.

Speaker: Ms Janet Chang (The University of Hong Kong)

Janet_Chang.pdf

17:00 Exploring Nuclear Gas Environments and Very Luminous Nuclear Transients Based on TDE Simulations

Directly resolving the gas distribution in galactic nuclei are challenging, but radio‑bright nuclear transients—especially tidal disruption events (TDEs)—offer a powerful indirect probe. Our three‑dimensional hydrodynamic simulations show that the interaction between a TDE outflow and the circumnuclear medium (CNM) produces radio light curves whose shapes reveal the CNM density profiles. Furthermore, bumpy gas or torus structures in the galactic nucleus imprint distinctive features on the radio emission. We also demonstrate that supernovae exploding near a supermassive black hole can undergo tidal distortion and circularization of their expanding ejecta, forming a new accretion disk and generating very luminous nuclear transients. These findings offer new approaches to exploring both the gas environment and stellar activity in galactic nuclei.

Speaker: Mr Xiangli Lei (Huazhong University of Science and Technology)

XiangLi_Lei.pdf

Wednesday 6 August

09:00 → 09:30 Morning Coffee

09:30 → 12:00 Changing-look AGNs: Benny Trakhtenbrot & Claudio Ricci

09:30 New and Puzzling Types of Transient Phenomena from Accreting Supermassive Black Holes

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, over surprisingly short timescales. These phenomena include “changing look AGN” and other, poorly understood flares from accreting SMBHs that show Bowen transitions driven by extreme UV radiation. I will discuss some of these new classes of transients, focusing on new results obtained with large time-domain surveys and responsive, multi-wavelength follow-up observations. While these events observationally differ from the (stellar) tidal disruption events known to date, the physics behind them may be interlinked. Together, these transient phenomena can greatly advance our understanding of SMBH accretion, teach us how and why SMBHs turn their accretion “on” and “off”, and reveal the sought-after signs of super-Eddington accretion. I will also mention how new & upcoming surveys are going to discover & survey many more SMBH-related transients.

Speaker: Prof. Benny Trakhtenbrot (Tel Aviv University)

Trakhtenbrot_CLAGN.pdf

10:30 Changing-State AGN and Rare AGN Transients

AGN are known to show flux variability over all observable timescales and across the entire electromagnetic spectrum. Over the past decade, a growing number of sources have been observed to show dramatic flux and spectral changes, both in the X-rays and in the optical/UV. Such events, commonly described as “changing-look AGN”, can be divided into two well-defined classes. Changing-obscuration objects show strong variability of the line-of-sight column density, mostly associated with clouds or outflows eclipsing the central engine of the AGN. Changing-state AGN are instead objects in which the optical/UV continuum emission and broad emission lines appear or disappear, and are typically triggered by strong changes in the accretion rate of the supermassive black hole. In addition to these, other rare classes of AGN transients have been discovered, including AGN flares potentially associated with extreme accretion events, or tidal disruption events occurring in active nuclei. The upcoming era of LSST is expected to significantly expand the discovery space for all these phenomena, thanks to its unparalleled cadence and depth. In my talk I will review our current understanding of Changing-state AGN, and then focus on future developments with LSST, particularly in combination with new-generation optical spectroscopic follow-up surveys (e.g. 4MOST).

Speaker: Prof. Claudio Ricci (University of Geneva)

Claudio_Ricci.pdf

12:00 → 12:30 AGNs: Chichuan Jin

12:00 NLS1

Speaker: Chichuan Jin (National Astronomical Observatories, Chinese Academy of Sciences)

12:30 → 13:30 Light lunch

13:30 → 18:30 Excursion

Self-funded, optional

18:30 → 20:30 Banquet

Thursday 7 August

09:00 → 09:30 Morning Coffee

09:30 → 10:30 AGNs: Jinyi Shangguan

09:30 VLTI/GRAVITY Interferometric Insights of AGN Inner Structure

Accurate supermassive black hole (SMBH) mass measurements are essential for understanding their formation and coevolution with host galaxies. Recent advances with the GRAVITY instrument on the Very Large Telescope Interferometer have revolutionized our ability to study active galactic nuclei (AGNs) and luminous quasars at unprecedented angular resolution. In this talk, I will present GRAVITY results spanning a wide redshift range. For low-redshift AGNs, our observations of ionized gas dynamics in the broad-line region (BLR) reveal a BLR radius–luminosity relation that is shallower than canonical. I will also report the first dynamical measurement of a high-redshift SMBH, showing that the single-epoch method overestimates the mass for this highly accreting SMBH, and that it appears undermassive relative to its host galaxy w.r.t. the local relation. I will conclude with prospects for GRAVITY+, whose enhanced adaptive optics and wide-angle phase referencing will expand our AGN sample and illuminate early SMBH growth.

Speaker: Prof. Jinyi Shangguan (Peking University)

Shangguan_Jinyi_online.pdf

10:30 → 11:00 Super-Eddington Accretion: Tom Kwan

10:30 Simulating Strongly Magnetized Super-Eddington Accretion Disk and Outflow Around Black Holes

Super-Eddington accretion happens in various astrophysical systems. For example, it is currently believed that super-Eddington accretion onto black holes powers tidal disruption events (TDEs), little red dots (LRDs) and a large fraction of ultra-luminous X-ray sources (ULXs). In this work, we conduct a series of 3D general relativistic radiation magnetohydrodynamics (GRRMHD) simulations of strongly magnetized accretion flows around stellar-mass black holes. While similar simulations have been performed previously, a systematic investigation of how various physical parameters affect the properties of the disk and outflow has been lacking. Our results disclose that the mass accretion rate and the black hole spin control the energy output and the outflow-inflow ratio for such accretion flows, which shed light on understanding super-Eddington systems such as LRDs. We address several questions, such as how luminous the emission sources can be, how beamed the emissions are, and what determines the power of their winds and jets.

Speaker: Mr Tom Kwan (The University of Hong Kong)

Tom_Kwan.pdf

11:00 → 11:20 AGNs: Rong Du

11:00 Measuring Accretion Disk Inclination and Mass Inflow Rates via Broadband SED Fitting

We present a unified study of inner accretion disk geometry and mass accretion rates in active galactic nuclei (AGNs) using multiwavelength nuclear spectral energy distribution (SED) fitting. First, by applying a self‑consistent X‑ray reflection model to joint XMM‑Newton and NuSTAR observations, we determine the inner disk inclination, $\theta_{\rm disk}$, with high precision—supported by extensive mock‐spectrum tests and the disk–BLR alignment demonstrated in our previous work. Then, using the retrieved inclination, we fit a three‑zone accretion model—comprising an inner slim disk, an outer truncated thin disk, and a hot corona—to derive the mass accretion rate. Together, these approaches establish broadband SED fitting as a robust diagnostic of disk inclination and accretion physics in AGNs, with important implications for black hole growth and feedback in both sub‑ and super‑Eddington regimes.

Speaker: Mr Rong Du

Rong_Du.pdf

11:20 → 11:40 X-ray Reflection: Zijian Zhang

11:20 Probing Super-Eddington Accretion via X-ray Reflection: Insights from Simulations and Observations

In this talk, I will explore the diagnostic power of X-ray reflection in unveiling the nature of super-Eddington accretion flows. In the first part, I present simulation results highlighting how reflection emission profiles are highly sensitive to wind kinematics and geometry. I show that incorporating multiple X-ray reflections within these winds can produce a distinctive double-peak line profile from face-on viewing angles. Furthermore, I demonstrate that the fluorescent Fe Kα line from super-Eddington flows occupies a parameter space distinct from standard thin disk accretion, offering a novel tool to identify and characterize black hole systems in super-Eddington states.
In the second part, I present a spectral analysis of EP240222a, an X-ray bright tidal disruption event (TDE) detected by the Einstein Probe. My modeling reveals a fast wind component with velocities reaching ~0.3c. This, alongside other observational signatures, supports the interpretation of EP240222a as a TDE from an intermediate-mass black hole that underwent super-Eddington accretion at its peak. Together, these findings underscore the potential of X-ray reflection studies to constrain extreme accretion phenomena in black hole systems.

Speaker: Mr Zijian Zhang

Zijian_Zhang.pdf

12:00 → 16:00 Free Discussion / Collaboration

Friday 8 August

09:00 → 09:30 Morning Coffee

09:30 → 10:30 Changing-look AGNs: Qingwen Wu

09:30 SMBH Transients and Broad-line Dynamics

More and more transients are found in the nuclear region of galaxies, such as tidal disruption event, changing-look AGNs, ambiguous nuclear transient etc. It is possible to learn the physical evolution of accretion, jet and BLR based on a single SMBH object now even though the physical mechanism triggers these transients may be still unclear. I will introduce: the possible physical mechanism for the changing-look AGNs based on multi-wavelength observations. The evolution of broad lines in changing-look AGNs and tidal disruption events will be also discussed based on theoretical and observational viewpoint.

Speaker: Prof. Wu Qingwen

QingWen_Wu.pdf

10:30 → 11:00 BFFs: A peculiar class of transients from accreting SMBHs

Speaker: Benny Trakhtenbrot (Tel Aviv University)

Trakhtenbrot_BFFs.pdf

11:00 → 11:30 TDEs: Lars Thomsen

11:00 Simulating Bowen Lines in Accretion Flow

Bowen lines, such as NIII4640 and NIII4100, were predicted to be detectable in supermassive black holes decades ago. However, they were only recently discovered in 2019 through extreme events such as tidal disruption events (TDEs), Changing Look AGNs, and Bowen Fluorescent Flare. The Bowen mechanism employs resonant absorption, EUV emission and outflows to account for the relatively strong line strengths of these Bowen lines without requiring super-solar nitrogen abundances.

In this talk, I will focus on simulating Bowen lines in TDEs, though the results can be generalized to any accretion flow. Utilizing a radiative transfer code within typical extreme accretion flow geometries, I will demonstrate how the different spectroscopic classes of TDEs are determined by the ionization level. I will also explain the conditions under which a Bowen-strong spectrum can be reproduced.

Speaker: Dr Lars Thomsen (The University of Hong Kong)

Lars_Thomsen.pdf

11:30 → 11:40 Closing Remarks