Analytical Model of Disk Evaporation and State Transitions in Accreting Black Holes
Apr 15, 202240 pages
Published in:
- Astrophys.J. 932 (2022) 2, 97
- Published: Jun 20, 2022
e-Print:
- 2204.07495 [astro-ph.HE]
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Abstract: (IOP)
State transitions in black hole X-ray binaries are likely caused by gas evaporation from a thin accretion disk into a hot corona. We present a height-integrated version of this process, which is suitable for analytical and numerical studies. With radius r scaled to Schwarzschild units and coronal mass accretion rate to Eddington units, the results of the model are independent of black hole mass. State transitions should thus be similar in X-ray binaries and an active galactic nucleus. The corona solution consists of two power-law segments separated at a break radius r
∼ 10(α/0.3), where α is the viscosity parameter. Gas evaporates from the disk to the corona for r > r
, and condenses back for r < r
. At r
, reaches its maximum, . If at r ≫ r
the thin disk accretes with , then the disk evaporates fully before reaching r
, giving the hard state. Otherwise, the disk survives at all radii, giving the thermal state. While the basic model considers only bremsstrahlung cooling and viscous heating, we also discuss a more realistic model that includes Compton cooling and direct coronal heating by energy transport from the disk. Solutions are again independent of black hole mass, and r
remains unchanged. This model predicts strong coronal winds for r > r
, and a T ∼ 5 × 10 K Compton-cooled corona for r < r
. Two-temperature effects are ignored, but may be important at small radii.Note:
- 40 pages, 12 figures, published in ApJ
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