X and UV radiation from accreting non-magnetic degenerate dwarfs
"X radiation from degenerate stars is produced by the impact of accreting matter with the stellar surface. The resulting X-ray and UV spectrum gives vital clues about the nature of the underlying star and the extreme conditions of temperature and magnetic field strength that may exist in the emission region. While the X-ray spectra of many sources have been extensively observed, the lack of systematic theoretical studies of X-ray emission from degenerate stars has so far prevented the exploitation of these observations. I have therefore carried out a comprehensive study of X-ray emission from non-magnetic degenerate dwarfs. I report the results of detailed numerical calculations that span the entire range of accretion rates and stellar masses, and that include the important, but previously unexplored, regime at moderate and high accretion rates. My calculations show that the characteristics of X radiation from non-magnetic degenerate dwarfs are qualitatively different than heretofore stated. I find: (1) Stars undergoing spherical accretion can produce a maximum hard X-ray luminosity Lhmax = 4.5 x 10^36 erg s-1 which is a factor of 4 larger, and occurs [or an electron scattering optical depth more than an order of magnitude larger, than earlier estimates suggested. (2) Contrary to previous reports, even high mass stars can produce low temperature (s 10 keY) X-ray spectra. (3) It has been stated that the temperature of the X-ray spectra produced by such stars is independent of X-ray intensity, and depends only on the stellar mass. I find, however, that at moderate and high accretion rates the spectral temperature varies dramatically and the star exhibits a pronounced-correlation between X-ray spectral temperature and luminosity. Such a correlation is not expected for neutron stars, and may be an important signature of degenerate dwarf X-ray sources. (4) At moderate and high accretion rates, the spectra have a power law high energy tail and, if the mass is high (M ~ 1. 2 Me) the spectra may even exhibit a more pronounced excess at high energies. Heretofore only neutron stars were thought able to produce high energy tails. I also find: (5) All spectra show an intense UV or soft X-ray blackbody component whose importance increases relative to the hard X-ray component as the accretion rate increases. (6) Inclusion of photoabsorption does not quantitatively alter the ""Eddington luminosity"" because the accreting gas is strongly photoionized by the intense UV or soft X-ray component. (7) At high accretion rates, absorption can produce a substantial low-energy cutoff in the hard X-ray spectrum. However, the absorption occurs far from the star and may, or may not, be present depending on the properties of the accreting gas far from the X-ray source. On the basis of the observed X-r3y spectrum, it has recently been claimed that Sco X-I cannot be a degenerate dwarf. In contrast, my calculations show that the spectra of a number of X-ray sources, including both Sco X-I and Cyg X-2, appear and show correlations between spectral temperature and intensity remarkably like that of degenerate dwarfs with hard X-ray luminosities Lh ~ 0.5 -5 x 10 36 erg s-l"
x radiation, UV radiation, accreting non-magnetic degenerate dwarfs, degenerate stars, stellar surface
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign