Discovery of the Neutron Star Spin Frequency in EXO 0748-676

Abstract

We report the results of a search for burst oscillations during thermonuclear X-ray bursts from the low mass X-ray binary (LMXB) EXO 0748-676. With the proportional counter array (PCA) onboard the Rossi X-ray Timing Explorer (RXTE) we detected a 45 Hz oscillation in the average power spectrum of 38 thermonuclear X-ray bursts from this source. We computed power spectra with 1 Hz frequency resolution for both the rising and decaying portions of 38 X-ray bursts from the public RXTE archive. We averaged the 1 Hz power spectra and detected a significant signal at 45 Hz in the decaying phases of the bursts. The signal is detected at a significance level of 4 x 10^-8. No similar signal was detected in the rising intervals. A fit to the oscillation peak at 0.25 Hz resolution gives a frequency of 44.7 +/- 0.06 Hz and an oscillation quality factor of Q = 80 +/- 18. The average signal amplitude is 3% (rms). The detection of 45 Hz burst oscillations from EXO 0748-676 provides compelling evidence that this is the neutron star spin frequency in this system. We use the inferred spin frequency to model the widths of absorption lines from the neutron star surface and show that the widths of the absorption lines from EXO 0748-676 recently reported by Cottam et al. are consistent with a 45 Hz spin frequency as long as the neutron star radius is in the range from about 9.5-15 km. With a known spin frequency, precise modelling of the line profiles from EXO 0748-676 holds great promise for constraining the dense matter equation of state.Comment: 13 pages, 5 figures, to be published in ApJ

Formation of Resonant Atomic Lines during Thermonuclear Flashes on Neutron Stars

“…Larger spin rates, assuming a NS radius of around 10 km, would wash out such a line unless the surface emission is confined to a narrow region around the spin axis ( Bhattacharyya et al 2004). A slow rotation rate for EXO 0748À676 is consistent with the recently measured 44.7 Hz burst oscillation ( Villarreal & Strohmayer 2004). With these line profiles in hand, and assuming that the burst oscillation of 44.7 Hz is the rotation rate of the NS, detailed fits to the data raise the exciting prospect of fitting for the NS radius (Chang et al 2005).…”

The Shapes of Atomic Lines from the Surfaces of Weakly Magnetic Rotating Neutron Stars and Their Implications

“…We confirm the finding of Villarreal & Strohmayer (2004), that the widths of these features are consistent with formation at the surface of a star with a spin rate of 45 Hz. We have also shown that stars with spin rates up to $600 Hz can produce lines this narrow if emission from close to the rotational axis dominates.…”

Does Submillisecond Pulsar XTE J1739−285 Contain a Weak Magnetic Neutron Star or Quark Star?

“…First, it is detected in the brightest burst with significance at the 99.96% confidence level, not in a whole set of six bursts (Kaaret et al 2007). Second, from a statistical point of view, the frequency of 1122 Hz is too far outside the range of the previously known spin frequencies of LMXBs, from the minimum value of 45 Hz (Villarreal & Strohmayer 2004) to the maximum of 619 Hz (Hartman et al 2003), centered at ∼400 Hz , and it is much higher than the fastest spin frequency of radio pulsars, 716 Hz (Hessels et al 2006). Therefore, we take this spin frequency of 1122 Hz as a tentative detection, or the possibility might be considered that the signal of 1122 Hz is merely a candidate burst oscillation (e.g., Galloway 2007 By means of this assumed highest spin frequency, we can constrain the NS mass and radius, which indicates the frequency of 1122 Hz to be close to the centrifugal breakup limit for some equations of state of nuclear matter (e.g., Burgio et al 2003).…”