Nearly coherent oscillations in type I X-ray bursts from KS 1731-260

We present an analysis of the full set of Rossi X-Ray Timing Explorer (RXTE) observations of KS 1731-260 made between 1996 August and 1999 February, concentrating on the nine type I X-ray bursts that were observed. We find that the bursts divide naturally into two populations: "fast bursts" occur on the Banana Branch when the accretion rate is high and exhibit short decay times, high peak fluxes, and radius expansion episodes. "Slow bursts" occur in the Island State at lower accretion rates, have lower peak fluxes, higher fluences, longer decay times, and show no evidence of radius expansion. All five of the fast bursts, and none of the four slow bursts, show coherent oscillations near 524 Hz. Thus, the mechanism that produces the burst pulsations may well be related to the helium-rich burning process indicated by the other properties of the fast bursts. We perform in-burst phase connection of the burst pulsations, which allows us to unambiguously characterize their frequency evolution. That evolution exhibits a variety of behaviors, including a sharp spin-down during one burst. The frequency evolution exhibited by two bursts that occurred 2.6 yr apart is consistent with nearly the same exponential-relaxation model; in particular, the asymptotic frequencies of the two bursts differ by only 0.13 +/- 0.09 Hz. The evolution during another burst, not modeled as exponential, shows a maximum frequency which is 0.6 +/- 0.2 Hz greater than the smaller of these asymptotic frequencies. Applying our phase models, we find that the pulsations are spectrally harder than the burst emission, with the strength of the pulsations increasing monotonically with photon energy. Coherently summing all of the burst signals, we find upper limits of similar to5% relative to the power of the main pulse on any modulation at 1/2 or 3/2 the main pulse frequency. We discuss our results in the context of models in which the burst pulsations are produced by a hot spot on the surface of a spinning neutron star.