Seminar by Mr. Ho Sang CHAN (University of Colorado)

Name: Seminar by Mr. Ho Sang CHAN (University of Colorado)
Start: 2026-02-12T16:30:00+08:00
End: 2026-02-12T17:15:00+08:00
Location: Rm G25, Science Centre North Block, The Chinese University of Hong Kong

Schedule

Thu Feb 12 2026 at 04:30 pm to 05:15 pm

UTC+08:00

Location

Rm G25, Science Centre North Block, The Chinese University of Hong Kong | Hong Kong, HK

Advances in computational technology have enabled astrophysicists to model accreting black holes with more detailed microphysics and over longer durations. I will talk about simulating accreting black holes to examine several largely unexplored topics.
First, I will describe our attempt to include electron-positron pairs in global, general-relativistic magnetohydrodynamic simulations of accreting black holes. We found that in models with moderate to high accretion rates, pairs are close to equilibrium near the disk midplane, where the scattering optical depth is high and pair-equilibrium timescales are short, and could be comparable to the Coulomb collision timescale. This suggests the possibility of a pair-regulated coronal temperature. In contrast, the upper corona and jets, where the scattering optical depth is relatively low and pair-equilibrium timescales are long, are populated with pairs that may exceed their equilibrium value by orders of magnitude. These pairs are transported outward by advection from the disk, which dominates over local pair processes. These results highlight the role of electron-positron pair physics in the accreting plasma around stellar‑mass and supermassive black holes.
Then, I will present our self-consistent simulation of a strongly relativistic tidal disruption event (TDE) with astrophysically relevant parameters. The simulation begins with the initial stellar approach and follows the debris evolution beyond the peak mass-return time. We find that general relativistic effects drive violent pericenter nozzle shocks and stream self-intersection shocks that rapidly dissipate orbital energy and reduce the fluid eccentricity, but this phase is short-lived. Continuous-stream self-intersections impart angular momentum to the incoming debris, thereby increasing their pericenter distances. The incoming stream experiences weaker relativistic effects, diminishing both the shock strength and the overall energy dissipation rate. The debris remains highly eccentric, with most of the returning mass residing near the orbital apocenter. Disk formation proceeds 10-20 times more slowly than conventionally assumed. The resulting hydrodynamics resemble those of weakly relativistic TDEs. These findings suggest that the influence of relativistic effects on the long-term evolution of TDEs is limited.