Observation of Kilohertz Quasi-periodic Oscillations from the Atoll Source 4U 1702−429 by the [ITAL]Rossi X-Ray Timing Explorer[/ITAL

Abstract

We present results of Rossi X-Ray Timing Explorer observations of the atoll source 4U 1702-429 in the middle of its luminosity range. Kilohertz-range quasi-periodic 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 of 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, superposed 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 with other atoll sources with low-frequency QPOs.