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

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.

💡 Research Summary

The authors report the discovery of a 552 Hz burst oscillation in the low‑mass X‑ray binary EXO 0748‑676 using data from the Rossi X‑ray Timing Explorer (RXTE). Two thermonuclear (type‑I) bursts observed in January and December 2007 each displayed a highly significant signal during the burst rise. By performing overlapping Fast Fourier Transforms (FFTs) on 1‑s, 2‑s, and 4‑s windows (stepped by 0.25 s) and searching for excess power above 10 Hz, they found Leahy powers of 59.68 and 48.26 at frequencies of 552.02 Hz and 552.45 Hz, respectively. The single‑trial false‑alarm probabilities are 10⁻¹³ and 10⁻¹¹; accounting for all trials across 157 bursts yields an overall significance of ≈10⁻¹⁰ (∼6.3σ). Dynamic power spectra reveal a modest frequency drift of 1–2 Hz, a hallmark of burst oscillations in other sources.

The oscillations have a fractional rms amplitude of about 15 % and show a clear energy dependence, rising from ~10 % at 2–4 keV to ~20 % above 9 keV. No significant power is detected at the first or higher harmonics, indicating a nearly sinusoidal waveform. The authors also performed a systematic search for the previously reported 45 Hz signal (Villarreal & Strohmayer 2004) in the enlarged sample of 157 bursts. While the original 38‑burst analysis reproduces the 45 Hz peak, the expanded dataset shows no statistically significant detection, even when restricting to bursts occurring during low persistent flux intervals. This casts serious doubt on the reality of the 45 Hz feature.

Given the strong, repeatable detection of the 552 Hz signal, its frequency drift, high amplitude, and energy dependence, the authors argue that this oscillation traces the neutron‑star spin frequency to within a few hertz, as is the case for burst oscillations in other systems. Consequently, the neutron star in EXO 0748‑676 must be a rapid rotator, not a slow 45 Hz spinner. This has important implications for the narrow absorption lines previously reported in XMM‑Newton spectra. At a spin of ~552 Hz, Doppler broadening would smear any surface‑origin lines to widths far larger than observed, implying that those lines cannot arise from the neutron‑star photosphere. Their origin must therefore lie elsewhere, such as in the accretion disk atmosphere or a boundary layer.

In summary, the paper provides compelling evidence that EXO 0748‑676 spins at ~552 Hz, overturning earlier claims of a 45 Hz spin. The detection solidifies the link between burst oscillations and neutron‑star rotation, and it challenges interpretations of surface spectral features in this source, prompting a re‑evaluation of line‑forming regions in rapidly rotating low‑mass X‑ray binaries.