Nearly Coherent Oscillations in Type I X‐Ray Bursts from KS 1731−260

We present an analysis of the nine type I X-ray bursts that were observedfrom KS 1731-260 with RXTE. We find that the bursts divide naturally into twopopulations: ``fast bursts'' occur on the Banana Branch when the accretion rateis high and exhibit short decay times, high peak fluxes, and radius expansionepisodes. ``Slow bursts'' occur in the Island State at lower accretion rates,have lower peak fluxes, higher fluences, longer decay times, and show noevidence of radius expansion. All five of the fast bursts, and none of the fourslow bursts, show coherent oscillations near 524 Hz. We perform in-burst phase connection of the burst pulsations, which allows usto unambiguously characterize their frequency evolution. That evolutionexhibits a variety of behaviors, including a sharp spin-down during one burst.Applying our phase models, we find that the pulsations are spectrally harderthan the burst emission, with the strength of the pulsations increasingmonotonically with photon energy. Coherently summing all of the burst signals,we find upper limits of ~5% relative to the strength of the main pulse on anymodulation at 1/2 or 3/2 the main pulse frequency. We discuss our results inthe context of models in which the burst pulsations are produced by a hot spoton the surface of a spinning neutron star.Comment: Version to be published in ApJ v542, Oct 20, 2000. Only minor changes. 24 pages including 8 figure