Her X-1 Exiting Anomalous Low State
ATel #307; Patricia Boyd (NASA/GSFC and UMBC) Martin Still (NASA/GSFC and USRA) and Robin Corbet (NASA/GSFC and USRA)
on 14 Jul 2004; 21:13 UT
Credential Certification: Patricia T. Boyd (padi@dragons.gsfc.nasa.gov)
Subjects: X-ray, Request for Observations, Binary, Neutron Star, Pulsar
Recent RXTE All-Sky Monitor (ASM) measurements of the X-ray flux of Her X-1
(= HZ Her) suggest that the source has exited the anomalous low state (ALS) and is
again exhibiting long-term variability with a period near 35 days. Current
(MJD 53196.8) RXTE ASM average count rates are above 5 counts/s,
whereas during ALS the ASM rate is typically below 2 counts/s at every
long term phase. (One Crab=75 counts/s in the ASM). Preliminary analysis
of a pointed RXTE Proportional Counter Array (PCA) observation obtained
2004-July 12 finds the source to be significantly brighter than during an ALS, with a
count rate of ~ 0.1 Crab (> 5 ASM counts/s) and large amplitude pulsations.
In addition, another weaker but marginally significant ASM
maximum was seen 35 days previously at peak time ~MJD 53163. This would be the
fourth time Her X-1 has been observed to return to large-amplitude long-term variability
after an anomalous low state. (During an ALS, the 35 day X-ray cycle turns off yet the 35-day
optical cycle continues unabated.) The 1.24-s pulse period of Her X-1, typically
spinning up, has been observed to spin down correlated with the behavior of ALS.
Analysis of the PCA data continues. Pointed observations over energy
bands from IR to hard X-rays, to investigate the coupling of the spin and (presumably)
accretion disk precession periods, are encouraged as the source evolves.
