Session:- Nuclear transients: TDEs, QPEs, and Changing-look AGNs.2
Theoretical and observational studies of astrophysical black holes have been in rapid development in recent decades. This conference will present major breakthroughs and key remaining questions concerning our fundamental understanding of astrophysical black holes, such as the formation and collimation of relativistic jets, the accretion and growth of black holes, the measurement of black hole spin, the merger of black holes, the first images of black hole shadow, the particle acceleration mechanisms and transient phenomena around black holes, and the feedback from massive black holes to galaxies.
◎Explanation for the late-time radio flares in tidal disruption event
Close encounters between stars and black hole result in the tidal disruption of the stellar due to the huge tidal force exerted on it, an accident known as Tidal Disruption Events(TDEs). Recently, a few tidal disruption events have shown late-time (years after discovery) radio flares that last hundreds of days with flux densities ranging from 1 to 100 mJy, such as ASASSN-15oi, AT2018hyz. Here the speaker and his research team propose a scenario that the late-time radio flares may originate from the interaction of the TDE outflow with circumnuclear dense gas clouds (torus). Collision of the outflow with the torus leads to the formation of the bow shock, which accelerates the electrons, amplifies the magnetic field, and produces a late-time radio flare after TDE. They calculate the associated radio signature and compare them with the observations. They find a general consistency between the observations and predictions with reasonable parameters value. Determined by the distance of the torus and the velocity of the outflow, the late-time radio flares occur hundreds of days after TDE and they can peak at 10³⁹ erg/s, which is comparable to the observation. And the rise time is about hundreds of days which depends on the radius of the torus and the velocity of the outflow. Observations will play a key role in unveiling the nature of the late-time radio flare and a systematic study of a much larger sample would help understand the process responsible for it.
時間：2023.06.25(日) 16:00~16:18
講者：Jialun Zhuang
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◎Tidal Stripping of a White Dwarf by an Intermediate-Mass Black Hole
During the inspiralling of a White Dwarf(WD) into an intermediate-mass black hole (∼10²˜⁵ M⊙), both Gravitational Waves(GWs) and ElectroMagnetic (EM) radiation are emitted. Once the eccentric orbit's pericenter radius approaches the tidal radius, the WD would be tidally stripped upon each pericenter passage. The accretion of these stripped mass would produce EM radiation. It is suspected that the recently discovered new types of transients, namely the quasi-periodic eruptions and the fast ultraluminous X-ray bursts, might originate from such systems. Modeling these flares requires a prediction of the amount of stripped mass from the WD and the details of the mass supply to the accretion disk. The research team and the speaker run hydrodynamical simulations to study the orbital parameter dependence of the stripped mass. They find that their results match the analytical estimate that the stripped mass is proportional to z⁵ˡ², where z is the excess depth by which the WD overfills its instantaneous Roche lobe at the pericenter. The corresponding fallback rate of the stripped mass is calculated, which may be useful in interpreting the individual flaring light curve in candidate EM sources. They further calculate the long-term mass-loss evolution of a WD during its inspiral and the detectability of the GW and EM signals. The EM signal from the mass-loss stage can be easily detected: the limiting distance is ∼320(Mh/10⁴ M⊙) Mpc for Einstein Probe. The GW signal, for the space-borne detectors such as Laser Interferometer Space Antenna or TianQin, can be detected only within the Local Supercluster (∼33 Mpc).
時間：2023.06.25(日) 16:18~16:36
講者：CHEN, Jin Hong (Sun Yat-Sen University)
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◎Compact objects around SMBHs: QPEs, FRBs and EMRIs
In this talk, the GW and EM radiation from a compact object orbiting an SMBH are discussed. First, the speaker will present a new formation channel of EMRIs with tidal disruption flares as EM counterparts. In this scenario, flares can be produced from the tidal stripping of the helium(He) envelope of amassive star by an SMBH. The remaining compact core of the massive star then evolves into an EMRI. Second,
Quasi-Periodic Eruptions(QPEs), a new kind of X-ray burst with a recurrence time of several hours, have been detected from SMBHs in galactic nuclei. QPEs can be generated from the Roche lobe overflows at each periapsis passage of an evolved star orbiting a SMBH. Third, the large rotation measures of some repeating FRBs indicate they are produced by neutron stars around SMBHs. In these above scenarios, low-frequency gravitational-wave EMRIs will be generated. The implication of joint detection of EMRIs and EM signals are discussed.
時間：2023.06.25(日) 16:36~16:54
講者：Prof. Fayin Wang (Nanjing University)
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◎The Transient Slim Disk of the Changing-look Active Galactic Nucleus 1ES 1927+654
In changing-look AGNs, the optical-to-X-ray continuum flux typically increases significantly as broad emission lines appear. The changing-look AGN 1ES 1927+654, hosting already a highly accreting black hole, displays peculiar X-ray properties after its optical changing-look event in early 2018. the research team and the speaker carried out a follow-up campaign to probe its extreme accretion physics, using 34 optical spectra, 800 NICER and 14 Swift/XRT observations, as well as 7 simultaneous XMM-Newton/NuSTAR exposures. Detailed spectral energy distribution analysis suggests that the black hole was accreting super-critically, with t−5/3 declining mass accretion rate. The bolometric luminosity was logarithmically dependent on the mass accretion rate, suggesting the existence of a slim disk. After 0.55M⊙ of material was consumed, the evolution of the radiation efficiency and disk temperature suggests that the accretion flow finally returned to a thin disk. The mass budget yields the radius of the most bound orbit, which is consistent with the broad-line region orbit, favoring a tidal disruption event as the origin of the outburst. During the transient slim disk phase, the X-ray corona tightly correlated with the properties of the inner accretion flow, suggesting that the corona plasma originated from the disk itself. Additionally, the UV-X-ray spectral index and bolometric correction follow a completely different branch during the slim disk phase. They model the X-ray variability with a Gaussian process, finding that the correlation timescale is tightly correlated with disk properties. This study provides several important insights into the physical properties of slim disks and the AGN unified model.
時間：2023.06.25(日) 16:54~17:12
講者：Ruancun Li (Peking University)
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地點：香港大學 百週年校園 CPD 3.04 Centennial Campus
語言：英語
是會議作坊，參加者必須登記，2023.04.30 截止
【此屬轉載訊息，以主事單位發佈為準】
此活動由香港大學物理系主辦。
https://astrobh.physics.hku.hk/event/3/timetable/#all.detailed
