Discovery of twin kHz quasi-periodic oscillations in the low-mass X-ray binary XTE J1701−407

Abstract

We report the discovery of kHz quasi periodic oscillations (QPOs) in three Rossi X-ray Timing Explorer observations of the low mass X-ray binary (LMXB) XTE J1701-407. In one of the observations we detect a kHz QPO with a characteristic frequency of 1153 +/- 5 Hz, while in the other two observations we detect twin QPOs at characteristic frequencies of 740 +/- 5 Hz, 1112 +/- 17 Hz and 740 +/- 11 Hz, 1098 +/- 5 Hz. All detections happen when XTE J1701-407 was in its high intensity soft state, and their single trial significance are in the 3.1-7.5 sigma range. The frequency difference in the centroid frequencies of the twin kHz QPOs (385 +/- 13 Hz) is one of the largest seen till date. The 3-30 keV fractional rms amplitude of the upper kHz QPO varies between ~18 % and ~30 %. XTE J1701-407, with a persistent luminosity close to 1 % of the Eddington limit, is among the small group of low luminosity kHz QPO sources and has the highest rms for the upper kHz QPO detected in any source. The X-ray spectral and variability characteristics of this source indicate its atoll source nature.