A highly coherent 523.92 \pm 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 PCA on RXTE .
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 \sim 2 s , was highly coherent ( Q \gtrsim 900 ) , and had a pulse fraction ( ratio of sinusoidal amplitude to mean count rate ) of 6.2 \pm 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 where the pulsations are associated with photospheric contraction .
These coherent signals may be interpreted as a direct indication of the neutron star spin .