Discovery of a 552 Hz burst oscillation in the low-mass X-ray binary EXO 0748-676

TL;DR

This paper reports the detection of 552 Hz burst oscillations in the neutron star EXO 0748-676, confirming its spin frequency and challenging previous interpretations of spectral features and earlier oscillation signals.

Contribution

First detection of 552 Hz burst oscillations in EXO 0748-676, establishing its neutron star spin frequency and refuting earlier 45 Hz claims.

Findings

Confirmed neutron star spin frequency near 552 Hz.

Challenged previous 45 Hz oscillation interpretation.

Implications for spectral line broadening due to Doppler effects.

Abstract

We report the detection of pulsations at 552 Hz in the rising phase of two type-I (thermonuclear) X-ray bursts observed from the accreting neutron star EXO 0748-676 in 2007 January and December, by the Rossi X-ray Timing Explorer. The fractional amplitude was 15% (rms). The dynamic power density spectrum for each burst revealed an increase in frequency of approx. 1-2 Hz while the oscillation was present. The frequency drift, the high significance of the detections and the almost identical signal frequencies measured in two bursts separated by 11 months, confirms this signal as a burst oscillation similar to those found in 13 other sources to date. We thus conclude that the spin frequency in EXO 0748-676 is within a few Hz of 552 Hz, rather than 45 Hz as was suggested from an earlier signal detection by Villarreal & Strohmayer (2004). Consequently, Doppler broadening must significantly affect spectral features arising from the neutron star surface, so that the narrow absorption features previously reported from an XMM-Newton spectrum could not have arisen there. The origin of both the previously reported 45 Hz oscillation and the X-ray absorption lines is now uncertain.