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

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

Aug 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004head....8.4005v&link_type=abstract

American Astronomical Society, HEAD meeting #8, #40.05; Bulletin of the American Astronomical Society, Vol. 36, p.1205

Astronomy and Astrophysics

Astronomy

Scientific paper

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 type-I X-ray bursts. We calculated power spectra with 1 Hz frequency resolution for both the rising and decaying portions of 38 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 significance level of the signal is 4 × 10-8. No similar signal was detected in the rising intervals. The oscillation peak is unresolved at 1 Hz frequency resolution, indicating an oscillation quality factor of Q = ν 0 / Δ ν fwhm > 45, and 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 the system. We also 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 for a neutron star radius in the range 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.

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