Phase Resolved Spectroscopy of Burst Oscillations: Searching for Rotational Doppler Shifts

Abstract

X-ray brightness oscillations with frequencies from 300 - 600 Hz have been observed in six low mass X-ray binary (LMXB) bursters with the Rossi X-ray Timing Explorer (RXTE). These oscillations likely result from spin modulation of a hot region on the stellar surface produced by thermonuclear burning. If this hypothesis is correct the rotational velocity of the stellar surface, $\approx 0.1 c$, will introduce a pulse phase dependent Doppler shift such that the rising edge of a pulse should be harder (blue shifted) than the trailing edge (red shifted). Detection of this effect would both provide further compelling evidence for the spin modulation hypothesis as well as providing new observational techniques with which to constrain the masses and radii of neutron stars. In this work I present results of an attempt to search for such Doppler shifts.