Discovery in 4U 1636−53 of Two Simultaneous Quasi-periodic Oscillations near 900 H[CLC]z[/CLC] and 1176 H[CLC]z[/CLC

Abstract

Using the Rossi X-Ray Timing Explorer, we observed the low-mass X-ray binary 4U 1636-53 on 1996 February 28, April 23, and May 29. On February 28, two simultaneous quasi-periodic oscillations (QPOs) occurred at frequencies between 890 and 920 Hz and between 1150 and 1193 Hz. The rms amplitude and FWHM of the lower frequency QPO were 7.3% ± 0.4% and 26 ± 4 Hz, whereas those of the higher frequency QPO were 6.1% ± 0.8% and 64 ± 25 Hz. Averaged over minutes, the FWHM of the lower frequency QPO was sometimes as low as 4 Hz. The rms amplitudes of the QPOs increase with photon energy. The frequency of the lower frequency QPO increases, and its amplitude decreases, with accretion rate as inferred from the position of the source in the X-ray color-color diagram. The frequency difference between the two QPOs is consistent with being constant at 276 ± 10 Hz. The frequency of the higher frequency QPO is the highest so far observed in a low-mass X-ray binary. Assuming that this frequency is the orbital frequency of gas in stable Keplerian orbit around the neutron star, we derive upper limits on the mass and radius of the neutron star of 2.1 M☉ and 16.5 km. On April 23 we detected no kilohertz QPOs (95% confidence upper limits were typically 3%-5.5% rms in the frequency range 100-2000 Hz). On May 29 we detected a single 853-896 Hz QPO with an amplitude of ~7% rms that increases with photon energy and decreases with QPO frequency. There was no significant correlation between the frequency of this QPO and the count rate or spectral changes.