The hard X-ray emission spectra from accretion columns in intermediate polars

Abstract

We consider the hard (greater than 2 keV) X-ray emission from accretion columns in an intermediate polar system, GK Per, using a simple settling solution. The rate of photon emission per logarithmic energy interval can be fitted with a power law, E<SUP>-gamma</SUP>, with gamma approximately 2.0, in agreement with observations. This index is only weakly dependent on the mass accretion rate, dot-M, for dot-M in the range of a few times 10<SUP>16-18</SUP> g/s. The peak energy of the photon spectra (after photoelectric absorption) is expected to be E<SUB>p</SUB> approximately (5 keV) gamma<SUP>-1/3</SUP> (N<SUB>H</SUB>/10<SUP>23</SUP>/sq cm)<SUP>1/3</SUP> where N<SUB>H</SUB> is the hydrogen column density along the line of sight. The observed spectra of GK Per and possibly of V1223 Sgr suggest N<SUB>H</SUB> approximately 10<SUP>23</SUP>/sq cm. This large N<SUB>H</SUB> may be due to partially ionized preshock column material. Alternatively, we also consider absorption by the cool outer parts of an accretion disk. In this case the photoelectric absorption depth in the disk is a sensitive function of inclination. For GK Per the required inclination is approximately 83 deg. For mass accretion rates larger than a critical rate of approximately 10<SUP>18</SUP> g/s, X-ray emission from the column accretion is significantly affected by radiation drag. Although the mass accretion rate increases dramatically during outbursts, the observed hard (greater than 2 keV) X-ray luminosity will not rise proportionately. The slope and peak energy of the outburst spectra are only weakly affected. We conclude that the observed X-ray spectra can be explained by this simple analytic solution and that the production of hard X-rays from the accretion shock at the magnetic poles in the intermediate polars is in general agreement with the observations. However, since the X-ray emission and absorption depend on the mass accretion rate in a complicated manner, observed hard X-ray luminosities (greater than 2 keV) are not a good indicator of the mass accretion rate in the X-ray emitting regions.