Quasi-periodic oscillations (QPO) with frequencies of 350-1170 Hz have recently been observed in power spectra of countrate modulations in 12 X-ray binaries. These QPO have fractional root-mean square (rms) amplitudes of 1-20%, and quality factors of 10-200. In six sources a pair of QPO peaks have been observed simultaneously (4U 0614+091, 4U 1728-34, Sco X-1, GX 5-1, 4U 1820-30, and 4U 1636-53), with frequencies separated by 200-400 Hz. In 4U 0614+091 and 4U 1728-34 the frequency separation remains constant during excursions in QPO frequency by 200,Hz. QPO frequency is strongly correlated with count rate in 4U 1820-30, 4U 0614+091, and 4U 1728-34, and with mass accretion rate as inferred from the Z-track in GX 5-1 and Sco X-1. A 363,Hz oscillation seen during X-ray bursts in 4U 1728-34 has a frequency consistent with the difference between the pair of QPO peaks, and has been inferred to be the spin frequency of the neutron star.
We are involved in studies of these kilohertz QPO in collaboration with researchers at the Astronomical Insistute, University of Amsterdam , the Departments of Physics and Astronomy, University of Illinois at Urbana-Champaign, and the Center for Space Research, Massachusetts Institute of Technology.
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"Discovery in 4U 1636-53 of Two Simultaneous Quasi-periodic
Oscillations near 900 Hz and 1176 Hz",
Wijnands, R. A. A., van der Klis, M., van Paradijs, J., Lewin,
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