Fine-Tuning the Accretion Disk Clock in Hercules X-1

Abstract

Rossi X-Ray Timing Explorer all-sky monitor count rates from the X-ray pulsar Her X-1 began falling consistently during the late months of 2003. The source is undergoing another state transition similar to the anomalous low state of 1999. This new event has triggered observations from both space- and ground-based observatories. In order to aid data interpretation and telescope scheduling and to facilitate the phase connection of cycles before and after the state transition, we have recalculated the precession ephemeris using cycles over the last 3.5 yr. We report that the source has displayed a different precession period since the last anomalous event. Additional archival data from the Compton Gamma Ray Observatory suggests that each low state is accompanied by a change in precession period and that the subsequent period is correlatedwith accretion flux. Consequently our analysis reveals long-termaccretion disk behavior that is predicted by theoretical models of radiation-driven warping.