A spherical accretion model of cyg X-1

THE binary X-ray source Cyg X-1 is widely believed to be a black hole, and most current models for explaining its X-ray spectrum¹ assume an accretion disk. The reason for requiring a disk, rather than a spherical accretion flow, is that earlier investigations of the properties of spherically accreting black holes seemed to indicate that, for the mass transfer rates required to give the observed luminosities of 10³⁷-10³⁸ erg s^-1, the cooling time of the gas would be so short compared to the freefall time that the gas would arrive essentially cold at the Schwarzschild radius, and there being no solid surface to transform the infall kinetic energy into thermal energy, very little of this would be radiated away^2-4.