ROSSI-X-RAY-TIMING-EXPLORER DISCOVERY OF COHERENT MILLISECOND PULSATIONS DURING AN X-RAY BURST FROM KS 1731-260

A highly coherent 523.92 +/- 0.05 Hz periodic X-ray signal has been ob served during a type I X-ray burst from the low-mass X-ray binary syst em KS 1731-260 with the Proportional Counter Array on Rossi X-Ray Timi ng Explorer. The spectral evolution of the burst indicates photospheri c-radius expansion and contraction. The 524 Hz signal occurred at the end of the contraction phase, lasted for similar to 2 s, was highly co herent (Q greater than or similar to 900), and had a pulse fraction (r atio of sinusoidal amplitude to mean count rate) of 6.2% +/- 0.6%. KS 1731-260 is one of only three systems that have exhibited high-coheren ce millisecond oscillations during X-ray bursts and the first reported in which the pulsations are associated with photospheric contraction. These coherent signals may be interpreted as a direct indication of t he neutron star spin.