One-pole and two-pole X-ray emission in the AM Herculis binary BL Hyi.

This paper reports the results of extensive X-ray observations of BL Hyi = H0139-68 with the Exosat satellite when the source was in states of low and intermediate brightness. In the low state, X-rays were observed only between orbital phases phi = 0.0 and about 0.4, coincident with the orbital hump of cyclotron emission from the main pole. In the intermediate state, burstlike soft X-ray emission appeared in the faint phase between phi = 0.4 and 1.0. This emission originates from a separate region probably near the second pole. An increased accretion rate at the second pole naturally explains the HEAO-1 light curve in the pre-1983 high state which showed soft X-ray emission at all orbital phases. The ratio of integrated blackbody vs bremsstrahlung luminosity increased from the main-pole value L(bb)/L(brems) about 1 in the low state to an orbital mean of L(bb)/L(brems) about 5 in the high state. These results support current models of soft X-ray generation by the infall of dense filaments of matter onto the photosphere of the white dwarf.