The Energy Dependence of Millisecond Oscillations in Thermonuclear X‐Ray Bursts

Abstract

We examine the energy-resolved pulse profiles of 51 flux oscillations observed during the declines of thermonuclear X-ray bursts from accreting weakly magnetized neutron stars with the Rossi X-Ray Timing Explorer. In general, we find that (1) the amplitudes of the oscillations increase as a function of energy, and (2) the pulses observed in the higher energy bands arrive later than those in lower energy bands, although the magnitude of the energy dependence varies significantly from burst to burst. Averaging groups of bursts from individual sources, we demonstrate that fractional rms amplitudes of the oscillations increase as a function of energy by 0.25%-0.9% keV-1 between 3 and 20 keV and are as large as 20% in the 13-20 keV band. We also show that the average delays between pulses observed in the high- and low-energy bands are 0.002-0.007 cycles keV-1 between 3 and 20 keV. This amounts to total delays of 0.03-0.12 cycles between the lowest and highest energy bands or time delays that range from 100 to 200 μs. We model the oscillations as flux variations arising from temperature patterns on the surfaces of rapidly rotating neutron stars. In this framework, we find that the increase in the pulse amplitude with photon energy can be explained if the cooler regions on the neutron star emit in the lower energy bands, reducing the flux variations there. On the other hand, the Doppler shifts caused by the rapid rotation of the neutron star should cause the hard pulses to precede the soft pulses by about 0.05 cycles (100 μs), in contrast to the observations. This suggests that the photons originating from the stellar surface are reprocessed by a hot corona of electrons before they reach the observer.