EUVE Observations of Hercules X-1 During a Short High State Turn-On

Abstract

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. The low state portion of the EUV light curve (prior to the start of the Short High state) has similar properties as that following the end of the Short High state (Leahy and Marshall, 1999). This is evidence that the low state EUV emission is primarily due to EUV reflection from the companion star HZ Her. The EUV lightcurve during the Short High state is pulsed and closely resembles the average 2-12 keV X-ray Short High state lightcurve indicating that the EUV emission, like the X-ray emission, originates 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 spectrum and pulse shape are similar to that in the soft X-rays (0.1 - 1 keV). The most likely origin of the EUV emission is reprocessed X-rays from the inner edge of the accretion disk, and 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).