Rossi X‐Ray Timing ExplorerAll‐Sky Monitor Observations of the 35 Day Cycle of Hercules X‐1

Abstract

Her X-1 has been monitored nearly continuously in soft X-rays (2-12 keV) since 1996 February by the All-Sky Monitor on board the Rossi X-Ray Timing Explorer. We present an analysis of these observations, which include 23 contiguous 35 day cycles. We present the best-yet average 35 day cycle light curve for Her X-1. Thirty-five day light-curve 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 phases 0-0.31, and the short high state covers phases 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 preeclipse absorption dips progressively become shallower during the main high state. The marching preeclipse dip period is best determined from reanalysis of archival data as 1.651±0.002 days if the dip marching can be extrapolated from the main high state into the short high state or 1.646±0.003 using dips within single main high states alone.