Black Holes in Binary Systems: Observational Appearances

Abstract

The outward transfer of angular momentum of accreting matter can lead to the formation of a disk around the black hole. The structure and radiation spectrum of the disk depends, in the main, on the rate of matter inflow $\dot M$ into the disk at its external boundary. Dependence on the efficiency of mechanisms of angular momentum transport (connected with the magnetic field and turbulence) is weaker. If $\dot M = {10^{ - 9}} - 3 \times {10^{ - 8}} {M_ \odot }/yr$, the disk around the black hole is a powerful source of X-radiation with hv ~ 1–10 keV and luminosity L ~ 10³⁷–10³⁸ erg s⁻¹. If the flux of the accreting matter decreases, the effective temperature of radiation and the luminosity will drop. At the same time when $\dot M >{10^{ - 9}}{M_ \odot }y{r^{ - 1}}$, the optical luminosity of the disk exceeds the solar one. The main contribution to the optical luminosity of the black hole is due to the re-radiation of that part of the X-ray and ultraviolet energy which is initially produced in the central high temperature regions of the disk and which is then absorbed by the low temperature outer regions. The optical radiation spectrum of such objects must be saturated by the broad emission recombination and resonance lines. Variability is connected with the character of the motion of the black hole and the gas flow in binary systems and possibly with eclipses. For well defined conditions, the hard radiation can evaporate the gas. This can counteract the matter inflow into the disk and lead to autoregulation of the accretion.

If $\dot M \gg 3 \times {10^{ - 8}}\left( {M/{M_ \odot }} \right){M_ \odot }y{r^{ - 1}}$, the luminosity of the disk around the black hole is stabilized at the critical level of L _cr = 10³⁸ (M/M _⊙) erg s⁻¹. A small fraction of the accreting matter falls under the gravitational radius whereas the major part of it flows out with high velocities from the central regions of the disk. The outfiowing matter is opaque to the disk radiation and completely transforms its spectrum. As a consequence, a black hole in a supercritical regime of accretion can appear as a bright star with a strong outflow of matter.

This paper was presented by J. E. Pringle, in lieu of an invited talk by Ya. B. Zel’dovich. This paper is similar in form to the introductory part of a paper to be published in full in Astron. Astrophys.