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 observed during a type I X-ray burst from the low-mass X-ray binary system KS 1731-260 with the Proportional Counter Array on Rossi X-Ray Timing Explorer. The spectral evolution of the burst indicates photospheric-radius expansion and contraction. The 524 Hz signal occurred at the end of the contraction phase, lasted for ~2 s, was highly coherent (Q ≳ 900), and had a pulse fraction (ratio 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-coherence 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 the neutron star spin.