Evidence for Antipodal Hot Spots During X-Ray Bursts from 4U 1636−536

The discovery of high-frequency brightness oscillations in thermonuclearX-ray bursts from several neutron-star low-mass X-ray binaries has importantimplications for the beat frequency model of kilohertz quasi-periodicbrightness oscillations, the propagation of nuclear burning, the structure ofthe subsurface magnetic fields in neutron stars, and the equation of state ofhigh-density matter. These implications depend crucially on whether theobserved frequency is the stellar spin frequency or its first overtone. Here wereport an analysis of five bursts from 4U 1636-536 which exhibit strongoscillations at approximately 580 Hz. We show that combining the data from thefirst 0.75 seconds of each of the five bursts yields a signal at 290 Hz that issignificant at the $4\times 10^{-5}$ level when the number of trials is takeninto account. This strongly indicates that 290 Hz is the spin frequency of thisneutron star and that 580 Hz is its first overtone, in agreement with otherarguments about this source but in contrast to suggestions in the literaturethat 580 Hz is the true spin frequency. The method used here, which is analgorithm for combining time series data from the five bursts so that thephases of the 580 Hz oscillations are aligned, may be used in any source tosearch for weak oscillations that have frequencies related in a definite way tothe frequency of a strong oscillation.Comment: 9 pages including one figure, uses aaspp4.sty, submitted to The Astrophysical Journal Letters on September 1