The white dwarf in AM Herculis.

Abstract

We present orbital phase-resolved UV spectroscopy of AM Herculis in its high and low state obtained with the IUE satellite. We derive the effective temperatures and sizes of the accretion-heated polecap in both states. We also estimate the effective temperature and radius of the white dwarf. The spot is hotter in the high state, but always covers a large fraction f~0.1 of the white-dwarf surface. We discuss the energy balance of accretion-induced irradiation of the white dwarf and re-emission in the UV. From two quasi-simultaneous sets of IUE and ROSAT observations, we find that the UV flux is consistent with being produced by reprocessing of hard X-ray and cyclotron emission from the hot post-shock plasma. The bulk of the soft X-ray emission does not enter this energy balance. Our finding partly resolves the soft X-ray puzzle. We re-determine the distance to AM Her to be d=91^+18^_-15_pc. The low-state accretion rate is ~10^-11^M_sun_/yr.