Stability of cyclotron line in Her X-1

Four observations now of Her X-l from RXTE show steadiness of the cyclotron line centroid which limits variability to about one percent, which is well less than the 10 percent variability characteristic of the other spectral parameters. This confines the average height of scattering above the polar cap to within 35 meters, and is consistent with models which put the radiat ing structure within meters of the surface of the neutron star. But in othe r pulsars observed line centroid changes have been correlated with luminosi ty changes, and if interpreted as variability of the height at which scatte ring takes place, hundreds of meters are required. These RXTE data are not yet in conflict with such an extended radiating structure.