Discovery of two simultaneous kilohertz quasi-periodic oscillations in KS 1731-260

We have discovered two simultaneous quasi-periodic oscillations (QPOs) at 898.3 +/- 3.3 Hz and 1158.6 +/- 9.0 Hz in the 1996 August 1 observation of the low-mass X-ray binary KS 1731-260 with the Rossi X-ray Timing Explorer. The rms amplitude and FWHM of the lower frequency QPO were 5.3 +/- 0.7% and 22 +/- 8 Hz, whereas those of the higher frequency QPO were 5.2 +/- 1.0% and 37 +/- 21 Hz. At low inferred mass accretion rate ((M) over dot) both QPOs are visible, at slightly higher (M) over dot the lower frequency QPO disappears and the frequency of the higher frequency QPO increases to similar to 1178 Hz. At the highest inferred (M) over dot this QPO is only marginally detectable (2.1 sigma) near 1207 Hz, which is the highest frequency so far observed in an X-ray binary. The frequency difference (260.3 +/- 9.6 Hz) between the QPOs is equal to half the frequency of the oscillations observed in a type I burst in this source (at 523.92 +/- 0.05 Hz; Smith, Morgan, & Bradt 1997). This suggests that the neutron star spin frequency is 261.96 Hz (3.8 ms), and that the lower frequency QPO is the beat between the higher frequency QPO, which could be a preferred orbital frequency around the neutron star, and the neutron star spin. During the 1996 August 31 observation we detected an additional QPO at 26.9 +/- 2.3 Hz, with a FWHM and rms amplitude of 11 +/- 5 Hz and 3.4 +/- 0.6%.