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M. Still, P. Boyd (NASA GSFC)
During the last months of 2003, the 35 day cycle of the X-ray pulsar Hercules X-1 decayed. The object now resides in a deep low state, resembling a persistent ADC source. While the mechanism that forces state changes is almost certainly variations in accretion disk structure, the engine ultimately driving structural evolution remains unknown. Nevertheless, anomalous low states provide us with an opportunity to test and confirm three independent predictions. I present recent and archival RXTE and XMM-Newton data revealing that: 1) disk evolution is driven, at least partially, by the radiation pressure of reprocessed X-rays from the central source, 2) The companion star is a source of Compton reflected X-rays, and 3) the accretion flux from the (currently obscured) neutron star remains approximately constant during the low state. Monitoring of Compton reflection during several orbital cycles allows crude estimates to be made of accretion disk structure.
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The author(s) of this abstract have provided an email address for comments about the abstract: Martin.Still@gsfc.nasa.gov
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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.