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ATEL # 2094; D. K. Galloway (CSPA, Monash), D. Chakrabarty (MIT), J. R. Lin (MIT)
on 23 Jun 2009; 7:05 UT
Password Certification: Duncan K. Galloway (firstname.lastname@example.org)
Subjects: X-ray, Binaries, Neutron Stars, Transients
We report the detection of millisecond oscillations in the rising phase of two thermonuclear (type-I) X-ray bursts from the low-mass X-ray binary EXO 0748-676, using data taken with the Proportional Counter Array (PCA) on the Rossi X-ray Timing Explorer (RXTE). In a burst on 2007 January 14 14:08:44 UT we detected oscillations at a frequency of 552.02 Hz in a 1-s segment of data, with a Leahy power of 59.7. In a second burst on 2007 December 13 13:29:20 UT we detected oscillations at a frequency of 552.45 Hz in a 2-s segment of data, with a Leahy power of 48.3. The estimated significance of these two detections, given the number of trials in each Fourier power spectrum, is 6.2 and 5.1 sigma, respectively. The full PCA energy range of 2-60 keV was used for the lightcurve for both cases. Taking into account the small difference in the signal frequencies, and the 6.9e8 trials (from searching overlapping 1-, 2- and 4-s windows covering each of 157 bursts from EXO 0748-676 in public RXTE data in the frequency range 10-4000 Hz), we calculate a combined null hypothesis probability for the two detections of 1e-10, equivalent to 6.3 sigma. We undertook simulations to determine the effect of deadtime as well as the non-constant X-ray light curve of the bursts, and confirm the theoretical significance estimates.
The rms pulse fraction in the two bursts was approximately 17%. We calculated a dynamic power density spectrum for each burst and observed an increase in frequency of ~1-2 Hz over the time during which the oscillation was present. The frequency increase (characteristic of burst oscillations), the high significance of each detection and the almost identical 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.
However, the frequency of the newly discovered oscillation is much higher than the 45 Hz signal reported in bursts from the same source by Villarreal & Strohmayer (2004, ApJL 614, L121). That signal was detected only by summing power spectra from early in the decay of 38 bursts, whereas the 552 Hz signal is sufficiently strong to be detected in individual bursts. Thus, we suggest that the neutron star is more likely spinning at the higher frequency. This result has important implications for interpreting the X-ray absorption lines previously reported in XMM-Newton spectra of bursts from this source (Cottam et al. 2002, Nature 420, 51).