EUVE Observations of Hercules X-1

Observations of Hercules X-1 by the Extreme Ultraviolet Explorer (EUVE) covering low state and the early part of the Short High state are reported here. This is the first EUV observation of this part of the 35-day cycle of Her X-1. Prior to the turn-on of Short High state, the EUV emission resembles that at the end of Short High state (Leahy & Marshall, 1999, ApJ 521, 328). This is further evidence that the low state EUV emission from Her X-1 is due to X-ray reflection from HZ Her. After turn-on to Short High state the EUV emission has the same orbital phase dependence as the X-ray emission and is pulsed thus indicating an origin near the neutron star. The Short High state EUV spectrum is consistent with a blackbody of temperature 0.13 keV and radius 230 km. The Short High state EUV emission and pulse shape observed here are similar to the EUV emission observed during the Main High state flux decline, and similar to the behavior in soft X-rays (0.1 - 1 keV). The EUV emission is most likely due to reprocessed X-rays from the inner edge of the accretion disk. The radius of the inner edge of the accretion disk is likely to be small, consistent with that determined from analysis of the X-ray pulse shape evolution (Scott, Leahy and Wilson, 2000 accepted). Acknowledgement: this work supported in part by the Natural Sciences and Engineering Research Council of Canada.