RXTE Observations of Hercules X-1 During the July 1998 Short-high State

Abstract

We present RXTE monitoring of the eclipsing X-ray binary Hercules X-1 conducted over the short-high state of July 1998. This was one of the last major short-high states before the source entered an anomalous low-state of activity. A comparison with previous epochs finds no evidence for special behavior during these observations. We determine orbital and pulsar spin periods to facilitate measurements period derivatives during the subsequent anomalous low state and during the next epoch of high-state activity. Spectrally, the decay of the short-high state and concurrent pre-eclipse dips are consistent with obscuration of a central X-ray source by a cloud of non-uniform column density. The standard model of a warped accretion disk of finite vertical scale height fits the characteristics of this absorber well. Pre-eclipse dips have durations a factor of a few longer than the characteristic durations of dips during main-high states. Pulse profile structure increases in complexity towards the tail of the short-high state suggesting changes in accretion curtain geometry.