# Accretion into black hole, and formation of magnetically arrested accretion disks

## TL;DR

This paper derives exact solutions for magnetic field growth during black hole accretion, explores the formation of magnetically arrested disks, and analyzes their radiation spectra, highlighting high radiative efficiency and observational signatures.

## Contribution

It presents new analytical solutions for magnetic field evolution and disk formation in black hole accretion, including the structure, shock parameters, and spectra of magnetically arrested disks.

## Key findings

- Magnetically arrested disks (MAD) have very high radiative efficiency (~0.5 M-dot c^2).
- The disk spectrum varies significantly between laminar and turbulent states.
- Observable signatures include infrared emission from turbulent disks and potential galactic BH candidates.

## Abstract

The exact time-dependent solution is obtained for a magnetic field growth during a spherically symmetric accretion into a black hole (BH) with a Schwarzschild metric. Magnetic field is increasing with time, changing from the initially uniform into a quasi-radial field. Equipartition between magnetic and kinetic energies in the falling gas is established in the developed stages of the flow. Estimates of the synchrotron radiation intensity are presented for the stationary flow. The main part of the radiation is formed in the region $r \leq 7 r_g$, here $r_g$ is a BH gravitational radius. The two-dimensional stationary self-similar magnetohydrodynamic solution is obtained for the matter accretion into BH, in a presence of a large-scale magnetic field, when the magnetic field far from the BH is homogeneous and does not influence the flow. At the symmetry plane perpendicular to the direction of the distant magnetic field, the quasi-stationary disk is formed around BH, which structure is determined by dissipation processes. Parameters of the shock forming due to matter infall onto the disk are obtained. The radiation spectrum of the disk and the shock are obtained for the $10\,\, M_\odot$ BH. The luminosity of such object is about the solar one, for a characteristic galactic gas density, with possibility of observation at distances less than 1 kpc. The spectra of a laminar and a turbulent disk structure around BH are very different. The turbulent disk emits a large part of its flux in the infrared. It may occur that some of the galactic infrared star-like sources are a single BH in the turbulent accretion state. The radiative efficiency of the magnetized disk is very high, reaching $\sim 0.5\,\dot M\,c^2$ so it was called recently as a magnetically arrested disk (MAD). Numerical simulations of MAD, and its appearance during accretion into neutron stars are considered and discussed.