Discovery of Two Simultaneous Kilohertz Quasi-periodic Oscillations in KS 1731-260

摘要

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 + - 0.7/100 and 22 + - 8 Hz, whereas those of the higher frequency QPO were 5.2 + - 1.0/100 and 37 + - 21 Hz. At low inferred mass accretion rate (M) both QPOs are visible, at slightly higher M the lower frequency QPO disappears and the frequency of the higher frequency QPO increases to ～1178 Hz. At the highest inferred M this QPO is only marginally detectable (2.1 σ) near 1207 Hz, which is the highest frequency so far observed in an X-ray binary.