Subjects: Astronomy >> Astrophysical processes submitted time 2023-06-07 Cooperative journals: 《天文学进展》
Abstract： The black hole accretion theory is an important theoretical basis of modern high-energy astrophysics, as well as an important theoretical tool for testing the law of mat#2;ter movement in the strong gravitational field around the black hole. The traditional black hole accretion model is suitable for low mass X-ray binaries(LMXB) in a strict sense, which is based on black hole binaries assuming that the accreted gas comes from the Roche lobe overflow (RLO) of the companion star. While for the high mass X-ray binaries (HMXB) represented by Cyg X-1 and the active galactic nuclei (AGNs) with supermassive black holes in the center, most of the accreted gas comes from diffuse media such as stellar wind, in#2;terstellar medium and torus, etc. Under such circumstances, the accretion flow at the outer boundary tends to have a much higher temperature, larger velocity dispersion and more stochastic angular momentum distribution than that of the LMXBs, which could create a different scenario of the accretion process. This is the basic assumption, and also, the inno#2;vation point of our diffuse media accretion model. In this model, the accretion flow appears as an ADAF-like hot gas when it enters the system. As the accretion flow moves towards the black hole, the increase of the gas density facilitates the Coulomb collisions between ions and electrons thus enhancing the radiation. Specifically, when the initial accretion rate is above a critical value (typically ˙m ≈ 0.02 ˙mEdd), the over-cooling of the corona in the inner region would lead to part of the coronal flow condensing into a cold disk, forming a hybrid two-component accretion flow from ISCO to a certain distance, defined as the condensation radius. Beyond this radius the accretion flow maintains as ADAF. This paper summarizes the recently-established diffuse media accretion model in details, including the physical basis of the model, the solution procedure of the equation set, as well as the numerically calculated structure and emergent spectrum of the accretion flow. Model application to two types of black hole systems, i.e. AGNs and HMXBs are also introduced. For AGNs, the correlations among some observable parameters (the X-ray photon index Γ- reflection scaling factor R relation and the hard X-ray photon index Γ2∼10 kev - Lbol/LEdd Eddington-ratio relation) can be generally reproduced by our model. For a typical HMXB, Cygnus X-1, the the#2;oretical derived values of photon counts and hardness approximately agree with the data in the hardness-intensity diagram(HID) for all MAXI observations during the time period from 2009 to 2018. Further theoretical development and application prospects are briefly summaried.
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