A Search for Variablity in the Cyclotron Resonance Scattering Feature in Hercules X-1

We propose to monitor the cyclotron line energy of the accreting X-ray pulsar Hercules X-1. A recent analysis of RXTE data, along with a spectral reanalysis of archival HEAO-1 observations, has show that the observed cyclotron energy increased by 20-percent sometime between 1991 and 1993. Furthermore, in the post-1993 data there is an indication of a decrease in energy of 0.3-keV/year. In AO7 we began a campaign to monitor the line centroid of Her X-1, the results of which are already intriguing. The proposed 4 more observations should be able to verify this movement at the 4.2-sigma level, with a third year increasing this to 4.8-sigma. Alternatively, if the current cyclotron energy is constant (or nearly so), we will be able to put a limit on the amount of variability.