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W. Coburn (SSL, UC Berkeley), W. A. Heindl (CASS, UC San Diego), R. Staubert, J. Wilms (IAA Tuebingen)
The accreting x-ray pulsar Hercules X-1 exhibits a spectral line-like feature in the hard x-rays that arises due to resonant scattering of photons out of the line of sight by electrons quantized into Landau levels by the magnetic field. This feature, known as a cyclotron line, is a direct measure of the magnetic field strength in the scattering region near the surface of the neutron star. A comparison of RXTE, BeppoSAX, and CGRO/BATSE data to archival observations, including a reanalysis of HEAO-1 data, has shown that the observed cyclotron line energy increased by approximately 20-percent between 1991 and 1993. The cause of the change is unknown, although it is possibly related to a change in the height of the scattering regions or a restructuring of the accretion mounds at the neutron star magnetic poles. In 2001 we started a campaign, using the RXTE, to monitor the line energy over time and watch for a return to the pre-1991 energy. Here we present the current status of those observations.
Bulletin of the American Astronomical Society,
© 2003. The American Astronomical Soceity.