Analytical Model of Disk Evaporation and State Transitions in Accreting Black Holes. (arXiv:2204.07495v2 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Cho_H/0/1/0/all/0/1">Hyerin Cho</a> (1 and 2), <a href="http://arxiv.org/find/astro-ph/1/au:+Narayan_R/0/1/0/all/0/1">Ramesh Narayan</a> (1 and 2) ((1) Center for Astrophysics | Harvard & Smithsonian, (2) Black Hole Initiative at Harvard University)
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 $dot{m}_c$ 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 AGN. The corona solution consists of two power-law segments
separated at a break radius $r_b sim10^3 ,(alpha/0.3)^{-2}$, where $alpha$
is the viscosity parameter. Gas evaporates from the disk to the corona for
$r>r_b$, and condenses back for $r<r_b$. At $r_b$, $dot{m}_c$ reaches its
maximum, $dot{m}_{c,{rm max}} approx 0.02, (alpha/0.3)^3$. If at $rgg
r_b$ the thin disk accretes with $dot{m}_d < dot{m}_{c,{rm max}} $, then the
disk evaporates fully before reaching $r_b$, 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 which includes Compton cooling and direct coronal heating
by energy transport from the disk. Solutions are again independent of black
hole mass, and $r_b$ remains unchanged. This model predicts strong coronal
winds for $r>r_b$, and a $Tsim 5times 10^8,{rm K}$ Compton-cooled corona
for $r < r_b$. Two-temperature effects are ignored, but may be important at
small radii.
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 $dot{m}_c$ 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 AGN. The corona solution consists of two power-law segments
separated at a break radius $r_b sim10^3 ,(alpha/0.3)^{-2}$, where $alpha$
is the viscosity parameter. Gas evaporates from the disk to the corona for
$r>r_b$, and condenses back for $r<r_b$. At $r_b$, $dot{m}_c$ reaches its
maximum, $dot{m}_{c,{rm max}} approx 0.02, (alpha/0.3)^3$. If at $rgg
r_b$ the thin disk accretes with $dot{m}_d < dot{m}_{c,{rm max}} $, then the
disk evaporates fully before reaching $r_b$, 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 which includes Compton cooling and direct coronal heating
by energy transport from the disk. Solutions are again independent of black
hole mass, and $r_b$ remains unchanged. This model predicts strong coronal
winds for $r>r_b$, and a $Tsim 5times 10^8,{rm K}$ Compton-cooled corona
for $r < r_b$. Two-temperature effects are ignored, but may be important at
small radii.
http://arxiv.org/icons/sfx.gif
