ROSAT OBSERVATIONS OF AN UMA AND MR SER - THE STATUS OF THE SOFT-X-RAY EXCESS IN AM HER STARS

We present the results of ROSAT pointed observations of the AM Her sta rs AN UMa and MR Ser. AN UMa was in a high-brightness state. We examin e its soft X-ray light curve, and fit its spectrum with a two-componen t model consisting of an absorbed soft blackbody and a harder thermal bremsstrahlung distribution. MR Ser was found to be in a low-brightnes s state, and had a spectrum which was well fitted with a single-compon ent, thermal bremsstrahlung model. The question of the energy balance in AM Her stars is reviewed with a view to quantifying the incidence o f the 'soft X-ray excess'. The excess has been explained as due to the deposition of energy directly into the white dwarf photosphere by the accretion stream, supplementing the energy of photons from the hot ac cretion column which irradiate the photosphere. We have calculated the energy balance in AN UMa and 16 other AM Her stars observed with ROSA T. We find that, although the majority of AM Her stars exhibit a soft X-ray excess, a number of objects are consistent with a model which in cludes radiative heating only. We examine possible factors which could determine the value of the soft X-ray excess. We find a correlation o f soft X-ray excess with magnetic field strength or, alternatively, th e radius at which material couples on to the magnetic field lines.