Ya. B. Zeldovich and foundation of the accretion theory

This short review is dedicated to academician Yakov Borisovich Zeldovich, the science of his epoch and the creation of modern accretion theory.Comment: 11 pages, 25 figures, accepted to special volume of Astronomy Report

On the appearance of non-local MRI in Keplerian accretion discs

We revisit the modal analysis of small perturbations in Keplerian ideal gas flows with constant vertical magnetic field leading to magneto-rotational instability (MRI) using the non-local approach. In the general case, MRI modes are described by a Schr\"odinger-like differential equation with some effective potential including 'repulsive' ($1/r^2$) and 'attractive' ($-1/r^3$) terms and are quantized. In shallow potentials, there are no stationary 'energy levels'. In thin Keplerian accretion discs, the perturbation wavelengths $\lambda=2\pi/k_z$ are smaller than the disc semi-thickness $h$ only in 'deep' potential wells. We find that there is a critical magnetic field for the MRI to develop. The instability arises for magnetic field below this critical value. In thin accretion discs, at low background Alfv\'en velocity $c_A\ll (c_A)_\mathrm{cr}$ the MRI instability increment $\omega$ is suppressed compared to the value obtained in the local perturbation analysis, $\omega\approx -\sqrt{3}\mathrm{i}c_Ak_z$. We also investigate for the first time the case of radially variable background magnetic field.Comment: 13 pages, 13 figures, to be submitted. Comments welcome

Accretion flows in astrophysics

This book highlights selected topics of standard and modern theory of accretion onto black holes and magnetized neutron stars. The structure of stationary standard discs and non-stationary viscous processes in accretion discs are discussed to the highest degree of accuracy analytic theory can provide, including relativistic effects in flat and warped discs around black holes. A special chapter is dedicated to a new theory of subsonic settling accretion onto a rotating magnetized neutron star. The book also describes supercritical accretion in quasars and its manifestation in lensing events. Several chapters cover the underlying physics of viscosity in astrophysical discs with some important aspects of turbulent viscosity generation. The book is aimed at specialists as well as graduate students interested in the field of theoretical astrophysics