Critical Accretion Disk

Abstract

For a supercritical accretion regime, we propose a critical accretion disk, where the mass-accretion rate is regulated just at the critical rate with the help of wind mass-loss. We first derive a critical radius, inside of which the standard picture is violated, using the condition that the radiative force is balanced by the gravity in the vertical direction. The critical radius $r_\mathrm{cr}$ is found to be $r_\mathrm{cr} = (9\sqrt{3}\sigma_\mathrm{T} / 16\pi c m_\mathrm{p}) \dot{M}_\mathrm{input} = 1.95 (\dot{M}_\mathrm{input} / \dot{M}_\mathrm{crit}) r_\mathrm{g}$, where $\dot{M}_\mathrm{input}$ is the mass-accretion rate at the outer edge of the disk, $\dot{M}_\mathrm{crit}$ the critical accretion rate, and $r_\mathrm{g}$ the Schwarzschild radius of the central object. Outside of this critical radius, the disk is in a radiation-pressure dominated standard state, while inside this radius the disk is in a critical state, where the excess mass is expelled by wind and the accretion rate is kept to be just at the critical rate at any radius inside $r_\mathrm{cr}$. In such a critical accretion disk, the disk thickness is $H \sim (1/6\sqrt{3})r$ and the surface temperature is $\sigma T^4 \sim (2/3\sqrt{3}) L_\mathrm{E} / 4\pi r^2$, where $L_\mathrm{E}$ is the Eddington luminosity. The total disk luminosity becomes $L_\mathrm{disk} \sim (2/3\sqrt{3}) [\ln (r_\mathrm{cr}/r_\mathrm{in})+1] L_\mathrm{E}$, where $r_\mathrm{in}$ is the inner radius. We apply the present model to microquasars and narrow-line Seyfert 1 galaxies, which are supposed to be under supercritical accretion.

The Physics of the “Heartbeat” State of GRS 1915+105

“…Thus, mapping LRH09's prescription for neutron stars onto GRS 1915+105, we can understand the gradual increase in the disk radius during the slow rise of the heartbeat state as a local Eddington phenomenon (our measured radii and temperatures are consistent with those predicted by Fukue 2004). The disk slowly expands with an increasing accretion rate.…”

“…LRH09 found that in the ensemble of observations in the vertex between the flaring branch and normal branch, the inner radius of the disk increased with luminosity above L ∼ 0.2 L Edd , while the disk temperature remained roughly constant. They interpreted this result as an indication of a local Eddington limit (Fukue 2004;Heinzeller & Duschl 2007) in the inner disk. Local Eddington effects arise because the radiation forces and gravitational forces depend differently on radius in thin disks, so that there exists a critical radius where radiation pressure can truncate the disk.…”

Radiation spectrum of a magnetized supercritical accretion disc with thermal conduction

“…As can be seen in figure 2, the maximum of νL ν is always of the order of the Eddington luminosity (L E = 10 39 ergs −1 ). This agrees with Fukue (2004) and observed value of LMC X-3. As we see by increasing of thermal conduction, the surface temperature decreases and therefore luminosity of disc also decreases.…”

Accretion Disks and Coronae in the X-Ray Flashlight

“…Ultimately, all three of these processes (outflow, inflow and heating) will be operating on an accretion disk during an X-ray burst, and it is unlikely that a single one dominates over the others (see also Fukue 1983;Walker and Meszaros 1989;Walker 1992;Fukue and Umemura 1995, for earlier discussions). Also, the effect on the corona cannot be fully decoupled: heating and outflows may truncate the disk, which can change the accretion rate and in turn affect both the disk and the corona.…”