THE HARD X-RAY-EMISSION SPECTRA FROM ACCRETION COLUMNS IN INTERMEDIATE POLARS

We consider the hard (>2 keV) X-ray emission from accretion columns in an intermediate polar system, GK Per, using a simple settling solutio n. The rate of photon emission per logarithmic energy interval can be fitted with a power law, E(-gamma), with gamma similar to 2.0, in agre ement with observations. This index is only weakly dependent on the ma ss accretion rate, M, for M in the range of a few times 10(16-18) g s( -1). The peak energy of the photon spectra (after photoelectric absorp tion) is expected to be E(p) similar to (5 keV)gamma(-1/3)(N-H/10(23) cm(-2))(1/3) where N, is the hydrogen column density along the line of sight. The observed spectra of GK Per and possibly of V1223 Sgr sugge st N-H similar to 10(23) cm(-2). This large N-H may be due to partiall y ionized preshock column material. Alternatively, we also consider ab sorption by the cool outer parts of an accretion disk. In this case th e photoelectric absorption depth in the disk is a sensitive function o f inclination. For GK Per the required inclination is similar to 83 de grees. For mass accretion rates larger than a critical rate of similar to 10(18) g s(-1), X-ray emission from the column accretion is signif icantly affected by radiation drag. Although the mass accretion rate i ncreases dramatically during outbursts, the observed hard (> 2 keV) X- ray luminosity will not rise proportionately. The slope and peak energ y of the outburst spectra are only weakly affected. We conclude that t he observed X-ray spectra can be explained by this simple analytic sol ution and that the production of hard X-rays from the accretion shock at the magnetic poles in the intermediate polars is in general agreeme nt with the observations. However, since the X-ray emission and absorp tion depend on the mass accretion rate in a complicated manner, observ ed hard X-ray luminosities (> 2 keV) are not a good indicator of the m ass accretion rate in the X-ray emitting regions.