The Energy Dependence of Millisecond Oscillations in Thermonuclear X-ray Bursts

We examine the energy-resolved pulse profiles of 51 flux oscillations observed during the decline of thermonuclear X-ray bursts from accreting weakly-magnetized neutron stars with the Rossi X-ray Timing Explorer. We find that the fractional rms amplitudes of the oscillations increase as a function of energy by 0.25% keV to 0.9% keV between 3−20 keV, and are as large as 20% in the 13−20 keV band. We also show that the pulses observed in the higher energy bands generally lag behind those in lower energy bands by 0.002 cycles keV to 0.007 cycles keV between 3− 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 100–200 μs. We then 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.