banner8 banner8_m

News and Events

UPCOMING EVENTS

BACK
Seminars

Public Seminar of PhD Candidate: Probing Winds in Super-Eddington Accretion via X-ray Reflection

Speaker

Mr. Zijian ZHANG

Affiliation The University of Hong Kong
Date July 9, 2025 (Wednesday)
Time 2:30 p.m.
Venue Room 518A, 5/F, Chong Yuet Ming Physics Building, The University of Hong Kong

Abstract

This dissertation is dedicated to investigating black hole extreme accretion and outflows through the novel technique of X-ray reflection. There are two major components for my study during the course of my PhD research.

 

For the first, main component of this dissertation, I developed a general relativistic Monte Carlo method and conducted a series of simulations to study X-ray reflections in winds launched from super-Eddington accretion disks. In particular, I incorporated multiple reflection physics, which has not been previously modeled for super-Eddington accretion flows. My results offered new insights into the behavior of X-ray reflection signatures from black hole systems with extreme accretion. I found that the reflection emission profiles are highly sensitive to the geometry and kinematics of the wind. Interestingly, I discovered that incorporating multiple X-ray reflections within winds can produce a novel spectral feature – a prominent double-peaked line profile – observed in face-on orientations. Moreover, I demonstrated that the fluorescent Fe Kα line – the most important signature of X-ray reflection – produced in super-Eddington accretion flows occupies a distinct parameter space compared to thin disks. These findings highlight the diagnostic potential of using X-ray reflection to identify black hole systems undergoing super-Eddington accretion and probe their properties.

 

For the second component of my study, I analyzed X-ray observations of super-Eddington black hole systems. In the thesis I only present the analysis I conducted for the reflection features observed from EP240222a, a distinctive X-ray strong tidal disruption event (TDE) discovered by the Einstein Probe. My spectral fitting revealed the presence of a fast wind with a velocity of approximately ~0.3c. This result, together with other supporting observational evidence, strongly suggests that this outburst is a TDE originating from an intermediate-mass black hole and reached super-Eddington state during the flare peak. This dissertation also highlights the essential and urgent need for advanced spectral modeling of super-Eddington accretion outflows, which is crucial for unlocking the full scientific potential of upcoming next-generation astronomical surveys.

 

Anyone interested is welcome to attend.