The Latitude of Type I X-Ray Burst Ignition on Rapidly Rotating Neutron Stars

We investigate the latitude at which type I X-ray bursts are ignited onrapidly rotating accreting neutron stars. We find that, for a wide range ofaccretion rates, ignition occurs preferentially at the equator, in accord withthe work of Spitkovsky et al. However, for a range of accretion rates below thecritical rate above which bursts cease, ignition occurs preferentially athigher latitudes. The range of accretion rates over which nonequatorialignition occurs is an increasing function of the neutron star spin frequency.These findings have significant implications for thermonuclear flamepropagation, and they may explain why oscillations during the burst rise aredetected predominantly when the accretion rate is high. They also support thesuggestion of Bhattacharyya & Strohmayer that non-photospheric radius expansiondouble-peaked bursts and the unusual harmonic content of oscillations duringthe rise of some bursts result from ignition at or near a rotational pole.Comment: 4 pages, 2 figures, accepted by ApJ