Cyclotron emission from white dwarf accretion columns

Abstract

We calculate the cyclotron absorption coefficient for a Maxwellian velocity distribution of electrons over a wide range of temperatures and frequencies following the method described by Chanmugam. These results are used to determine the range of parameters for a white dwarf accretion column in which a simple steady-state bombardment solution can be found, where the dominant radiative-loss mechanism is optically thick cyclotron radiation. We show that for an acceptable choice of accretion parameters a bombardment solution exists with a temperature ~10 keV, somewhat lower than the shock temperature Ts≃30 keV. We also demonstrate that for parameters for which a simple bombardment model is not valid, the electron temperature (and therefore the observed temperature) will never reach the shock temperature. This leads to a lower hard/soft X-ray ratio than that obtained from the adiabatic shock model – and could thus ease the soft X-ray problem associated with AM Herculis stars.