X-ray and UV radiation from accreting nonmagnetic degenerate dwarfs. II

Abstract

We report the results of detailed numerical calculations of X-ray and UV emission from accreting nonmagnetic degenerate dwarfs. The results are also valid for magnetic degenerate dwarfs so long as bremsstrahlung cooling is more important than cyclotron cooling. Our calculations span the entire range of accretion rates and stellar masses and include the important, but previously unexplored, regime at moderate and high accretion rates. We find that the maximum hard X-ray luminosity for degenerate dwarfs undergoing spherical accretion is 2.2 X10 36 ergs s-1. This maximum luminosity is little changed if accretion occurs radially over only a fraction of the stellar surface, so that the emitted radiation escapes without significant scattering. The temperature characterizing the X-ray spectra produced by degenerate dwarfs strongly depends not only on the stellar mass, as was previously thought, but also on the accretion rate. The resulting correlation between spectral temperature and luminosity may be an important signature of degenerate dwarf X-ray sources. We apply our calculations to cataclysmic variables such as AM Her and SS Cyg and discuss the implications of our results for Cyg X-2 and the Sco X-l-like sources.