Session:- Simulations of accretion and outflow near black holes.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.
◎Relativistic Jet Simulations and Modeling on Horizon Scale
Relativistic jets are launched in the vicinity of the central black holes and emit powerful radiation across the electromagnetic spectrum. According to current understanding of the research team, relativistic jets are launched by directly tapping the rotational energy of spinning black holes via the so-called Blandford-Znajek process. In addition to the spin of the black hole, numerical simulations showed the amount of accreted magnetized flux has a major impact on the formation of relativistic jets. They have investigated the radiative signatures of self-consistently launched relativistic jets using 3D general relativistic magneto-hydrodynamical simulations and general relativistic radiative transfer calculations in horizon scale to the connection with large-scale structure. They discuss their findings and comparison with observations.
時間：2023.06.24(六) 11:10~11:28
講者：Prof. MIZUNO, Yosuke (Tsung-Dao Lee Institute, Shanghai Jiao Tong University)
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◎Simulating Bondi-Like Accretion Flow Around Black Holes
While black hole accretion flow simulations usually start from gas disks with relatively large angular momenta, many important black hole astrophysical systems are believed to be fed by gas with low angular momenta, such as tidal disruption events, long gamma-ray bursts and wind-fed high mass X-ray binaries. The speaker and his team carry out 3D General Relativistic MagnetoHydroDynamic(GRMHD) simulations of accretion flows with zero or very low specific angular momenta around rapidly spinning black holes. They thread the flows with large amounts of large-scale ordered magnetic fields. The results show that such accretion flow needs to initially have a specific angular momentum above a certain threshold to eventually reach and robustly sustain the Magnetically Arrested Disk(MAD) state. If the flow can reach the MAD state, it can launch very powerful jets. Furthermore, they realize that even when the accretion flow has initial specific angular momentum below the threshold, it can still launch episodic jets with an average energy efficiency of ∼10%. However, the accretion flow has non-typical and interesting behaviours in this situation. Their results give an insight into the behaviour of the accretion flow and the production of relativistic jets in various astrophysical systems in which the accretion flows likely have low specific angular momenta.
時間：2023.06.24(六) 11:28~11:46
講者：KWAN, Tom (The University of Hong Kong)
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◎Can two-temperature treatments in GRMHD simulations reduce the predicted variability compared to historical observations of Sgr A*?
Recent direct imaging by the Event Horizon Telescope Collaboration(EHTC) has confirmed the existence of an event horizon in Sagittarius A(Sgr A), the supermassive black hole at the center of the Milky Way. To model and interpret these data, the EHTC compares them to a library of fiducial models based General Relativistic MagnetoHydroDynamic (GRMHD) simulations. One significant source of inaccuracy stems from the fact that the evolution equations model only a single-particle fluid with temperature associated with the proton temperature, and thus do not account for the electrons self-consistently. Instead, as an approximation, the EHTC uses a pre-determined prescription of the temperature ratio between electrons and protons typically based on the local magnetisation of the plasma. This approximation strongly affects the predicted emission, since the radiative transfer depends on the electron temperature assuming a thermal distribution function. One way these models can be tested is via variability of light curves at 230 GHz. Observations of Sgr A* spanning decades give very strong constraints on the variability, and so far all of the strongly magnetized models used by the EHTC show too much variability comparatively. Moreover, none of the models successfully pass all the variability and multiwavelength constraints together.
The temperature ratio between protons and electrons depends on a balance between microphysical dissipation not captured in ideal fluid simulations, radiative cooling, and fluid transport. Therefore, the speaker and his team investigate the effects of two-temperature thermodynamics of a magnetically arrested disk around Sgr A, where the temperatures of both species are evolved more self-consistently following a method by Ressler et al. (2015) and Sadowsky el al. (2017). They include Coulomb coupling, heating of non-thermal electrons via an assumed magnetic reconnection mechanism (Rowan et al. 2017) and radiative cooling. Simulations incorporating radiative cooling already show differences in the dynamical and geometrical properties of the accretion flow compared to fiducial models, but the inclusion of heating in addition and two-temperature thermodynamics has not been studied, particularly at higher resolution. These effects also depend strongly on accretion rate which is another unknown variable. In this talk the speaker will present their recent study of the impact of these effects on the predicted light curve and morphology of the accretion flow close to the black hole, and compare their results to the fiducial models used by the EHTC in its 2022 study of the first campaign on Sgr A.
時間：2023.06.24(六) 11:46~12:04
講者：Leon S. Salas
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◎Evolution of wandering intermediate-mass black holes in high-z galaxies with 3D radiation hydrodynamics simulations
The speaker and her team have, for the first time, successfully performed three-dimensional radiative hydrodynamics simulations of the gas accretion onto InterMediate-mass Black Holes(IMBHs) wandering in the high-z galaxies. Here the sublimation of the dust grain caused by the radiation from the accretion disks around IMBHs is taken into consideration. They found that the accretion rate and acceleration are ∼7×10⁻⁶ M⊙yr⁻¹ and ∼10⁻⁸cm s⁻² in environments with relatively high density (≥104cm⁻³) and low metallicity (0.1Z⊙). These results suggest that IMBHs keep floating in the galactic disk with insignificant mass growth. For extremely high density (≥106cm⁻³), as suggested by recent observations from the James Webb Space Telescope, the accretion rate and acceleration rise significantly. This indicates that IMBHs are ejected from the galactic disk due to an increase in mass and velocity.
時間：2023.06.24(六) 12:04~12:22
講者：OGATA, Erika (University of Tsukuba)
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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
