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Session 35 - Galactic X-Ray (XTE) Sources.
Display session, Tuesday, January 14
Metropolitan Ballroom,
We present the detection of a highly coherent 524.0 Hz periodic signal during a Type I X-ray burst from KS 1731-260 observed on July 14.178 UT with the PCA on RXTE. (A preliminary report of this work was made by Morgan and Smith in IAUC 6437.) From a persistent emission of 250 mCrab, the intensity rose in \sim0.5 seconds to 1.5 Crab. The total burst duration is \sim30 s, with a decay time scale of \sim5 s in the full PCA energy range of 2-60 keV. We present evidence from spectral fits that this burst exhibits photospheric radius expansion, and the 524.0 Hz signal is detected near the end of the contraction phase. The signal lasts for \sim1.5 s, is highly coherent (Q > 1048), and has a pulse fraction (ratio of pulsed amplitude to mean count rate) of 5.8\pm0.9%. This 524-Hz signal is similar in some respects to the 363-Hz signal found only during bursts from 4U 1728-34 (IAUC 6387) and the 589-Hz signal from the unidentified burster near GRO 1744-28 (IAUC 6484) reported by Strohmayer, et al. No periodic or quasiperiodic signals are detected in the persistent emission or in the tail of the burst. We speculate that we have detected the rotation frequency of the neutron star, in which case the spin period is 1.91 ms.
