Radiative effects in supersonic wind accretion onto gravitating objects

¹ Institute for Problems in Mechanics, Russian Academy of Sciences, 101-1 Vernadskii Avenue, 119526 Moscow, Russia e-mail: kryukov@ipmnet.ru
² Institute of Geophysics and Planetary Physics, University of California, Riverside, CA 92521, USA e-mail: nikolai.pogorelov@ucr.edu
³ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85741 Garching, Germany e-mail: [ula;grb]@mpa-garching.mpg.de
⁴ Space Research Institute, Russian Academy of Sciences, Profsoyuznaya St. 84/32, 117810 Moscow, Russia e-mail: kogan@mx.iki.rssi.ru

Received: 18 March 2005
Accepted: 24 June 2005

Abstract

We investigate the influence of radiative effects on supersonic wind accretion onto gravitating objects. The accreting matter is assumed to be optically thin. The physical mechanisms taken into account include cooling due to free-free and free-bound transitions, the Compton heating via X-ray scattering on electrons and the inverse Compton cooling in the regions where the temperature of the matter becomes sufficiently large to be able to transfer part of its internal energy to photons. A wide range of determining parameters was covered, including the values applicable to the Vela X-1 binary system, but our main emphasis is on the study of the effects of radiative processes on the behavior of accretion flows. It is shown that the applicability of polytropic accretion models is very limited and the actual accretion rate can be considerably lower than that provided by the Bondi–Hoyle–Lyttleton formula. The detailed consideration of the realistic radiative effects proved to be of great importance in our understanding of the accretion phenomenon, since they can substantially affect it both qualitatively and quantitatively.