Evidence for Antipodal Hot Spots During X-ray Bursts From 4U 1636-536

Abstract

The discovery of high-frequency brightness oscillations in thermonuclear X-ray bursts from several neutron-star low-mass X-ray binaries has important implications for the beat frequency model of kilohertz quasi-periodic brightness oscillations, the propagation of nuclear burning, the structure of the subsurface magnetic fields in neutron stars, and the equation of state of high-density matter. These implications depend crucially on whether the observed frequency is the stellar spin frequency or its first overtone. Here we report an analysis of five bursts from 4U 1636-536 which exhibit strong oscillations at approximately 580 Hz. We show that combining the data from the first 0.75 seconds of each of the five bursts yields a signal at 290 Hz that is significant at the $4\times 10^{-5}$ level when the number of trials is taken into account. This strongly indicates that 290 Hz is the spin frequency of this neutron star and that 580 Hz is its first overtone, in agreement with other arguments about this source but in contrast to suggestions in the literature that 580 Hz is the true spin frequency. The method used here, which is an algorithm for combining time series data from the five bursts so that the phases of the 580 Hz oscillations are aligned, may be used in any source to search for weak oscillations that have frequencies related in a definite way to the frequency of a strong oscillation.