Observations of the X-Ray Cyclotron Line Centroid of HER X-1

The X-Ray spectrum of Her X-1 during its high state is notable for the presence of a spectral feature near 35 keV which is generally interpreted as due to cyclotron absorption in a 10(13) Gauss magnetic field above the polar cap of the neutron star. The magnetic field strength changes quickly in this region. Thus if the structure of the radiating region should vary, say in response to a change in accretion rate, then the observed centroid should be a sensitive measure of such changes. Most existing measurements are consistent with an unvarying centroid to a precision of the order of only 15%, determined by counting statistics and intercalibration errors. The Rossi X-Ray Timing Explorer (RXTE) has observed the X-Ray binary Her X-1 on several occasions during the first six months of the mission. With its large area, HEXTE obtains a centroid near 37 keV with a statistical precision of about 2% during a typical orbit of observation and there have been many such orbits. Intercomparison of these centroid determinations appears to be limited only by counting statistics, thus the existing RXTE measurements should provide a greatly improved measure of the variability of the line centroid over time scales from 90 minutes to 8 months, or set greatly improved limits.