The thickness of a weakly magnetized accretion flow inside the last stable orbit of a Kerr black hole

Abstract

If accretion disc contains weak frozen-in entangled magnetic fields, their dynamical effect may be important inside the last stable orbit because of the decompression near the sonic point. Here, I consider the radial and vertical structure of a nearly free-falling flow inside the last stable orbit of a thin disc around a Kerr black hole. The thickness of such a flow is determined primarily by the vertical stress created by radial and azimuthal magnetic fields. The thickness is predicted to oscillate vertically around its equilibrium value determined by the magnetic field balance with gravity. For thin discs, this thickness is much larger than that of the accretion disc itself. Numerical simulations with HARM2d show the vertical structure is more complicated. In particular, magnetically supported disc seems to be unstable to segregation of matter into thinner streams with the vertical scale determined by thermal pressure or other processes.

The soft state of the black hole transient source MAXI J1820+070: emission from the edge of the plunge region?

“…Matter enters ballistic plunge orbits at that radius with very little dissipation or further emission as it falls into the black hole. Later work in which the effect of magnetic fields are considered has suggested that stresses can and do occur at the ISCO leading to dissipation and excess emission as matter enters the plunge region (Reynolds & Armitage 2001, Hawley & Krolik 2002, Machida & Matsumoto 2003, Shafee et al 2008, Zhu et al 2012, and Abolmasov 2014. One of the most detailed studies, by Zhu et al (2012) shows that further inward flow can cause a weak power-law-like continuum composed of ever hotter black body (bb) emission as matter falls further.…”

On the Eddington limit for relativistic accretion discs

“…We have considered the results of a thick disc MHD simulation from Abolmasov (2014), namely the run B1h presented in this work. Simulation was performed using the numerical code HARM2D that solves ideal MHD equations in Kerr metric (Gammie et al 2003;Noble et al 2006).…”

“…Numerical simulations of thick MHD discs support the assumption of Keplerian rotation in the inner parts of thick accretion discs and even inside the last stable orbit (Penna et al 2010). We have considered the results of a thick disc MHD simulation from Abolmasov (2014), namely the run B1h presented in this work. Simulation was performed using the numerical code HARM2D that solves ideal MHD equations in Kerr metric (Gammie et al 2003;Noble et al 2006).…”

“…Amplified magnetic fields lead to outward angular momentum transfer and consequently to accretion from the torus through a moderately thick disc formed during the first two thousand dynamical time-scales (GM/c 3 ) of the simulation. We also use the simulation run A50s presented in Abolmasov (2014) that was calculated for a = 0 and multi-loop initial magnetic field and uses smaller resolution. Main details of the simulations used in the present paper are given in Table 1.…”