RXTE/ASM Observations of the 35-day Cycle of Her X-1

Abstract

Her X-1 has been monitored nearly continuously in soft X-rays (2-12 keV) since February 1996 by the ASM on board RXTE. We present analysis of these observations which include 23 contiguous 35-day cycles. We present the best yet average 35-day cycle lightcurve for Her X-1. 35-day lightcurve features such as eclipses and absorption dips, and the alternating occurrence of the Main High and Short High states, are well measured. The occurrence of X-ray eclipse during the Low state is confirmed as is the presence of Main High states that ``turn-on'' sharply at orbital phases 0.23 and 0.68. The Main High state covers 35-day phase 0-0.31 and the Short High state covers phase 0.57-0.79, with phase 0 defined by Main High state turn-on. We find extended ingresses and egresses for the Short High state. Marching pre-eclipse absorption dips progressively become shallower during the Main High state. The marching pre-eclipse dip period is best determined from reanalysis of archival data as 1.651 +/- .002 days if the dip marching can be extrapolated from the Main High state into the Short High state or 1.646 +/- .003 using dips within single Main High states alone.