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

RXTE ASM count rates from the X-ray pulsar Her X-1 began falling consistentlyduring the late months of 2003. The source is undergoing another statetransition similar to the anomalous low state of 1999. This new event hastriggered observations from both space- and ground-based observatories. Inorder to aid data interpretation and telescope scheduling, and to facilitatethe phase-connection of cycles before and after the state transition, we havere-calculated the precession ephemeris using cycles over the last 3.5 years. Wereport that the source has displayed a different precession period since thelast anomalous event. Additional archival data from CGRO suggests that each lowstate is accompanied by a change in precession period and that the subsequentperiod is correlated with accretion flux. Consequently our analysis revealslong-term accretion disk behaviour which is predicted by theoretical models ofradiation-driven warping.Comment: 9 pages, 4 figures, ApJL in pres