EUVE Observations of Her X-1 at the End of the Short High State

Abstract

Observations of Her X-1 by the Extreme Ultraviolet Explorer (EUVE) at the end of the x-ray Short High state are reported here. Her X-1 is found to exhibit a strong orbital modulation of the EUV flux, with a large dip superposed on a broad peak around orbital phase 0.5 when the neutron star is closest the observer. Alternate mechanisms for producing the observed EUV lightcurve are modeled. We conclude that: i) the x-ray heated surface of the companion is too cool to produce enough emission; ii) the accretion disk can produce enough emission but does not explain the orbital modulation; iii) reflection of x-rays off of the companion can produce the shape and intensity of the observed lightcurve. The only viable cause for the large dip at orbital phase 0.5 is shadowing of the companion by the accretion disk.