Observation of Kilohertz Quasiperiodic Oscillations from the Atoll Source 4U 1702-429

Abstract

We present results of Rossi X-Ray Timing Explorer (XTE) observations of the atoll source 4U 1702-429 in the middle of its luminosity range. Kilohertz-range quasiperiodic oscillations (QPOs) were observed first as a narrow (FWHM ~7 Hz) peak near 900 Hz, and later as a pair consisting of a narrow peak in the range 625--825 Hz and a faint broad (FWHM 91 Hz) peak. When the two peaks appeared simultaneously the separation was 333 +/- 5 Hz. Six type I thermonuclear bursts were detected, of which five exhibited almost coherent oscillations near 330 Hz, which makes 4U 1702-429 only the second source to show burst oscillations very close to the kilohertz QPO separation frequency. The energy spectrum and color-color diagram indicate that the source executed variations in the range between the ``island'' and ``lower banana'' atoll states. In addition to the kilohertz variability, oscillations at ~10, ~35, and 80 Hz were also detected at various times, superimposed on a red noise continuum. The centroid of the \~35 Hz QPO tracks the frequency of the kilohertz oscillation when they were both present. A Lense-Thirring gravitomagnetic precession interpretation appears more plausible in this case, compared to other atoll sources with low frequency QPOs.