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 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, 1112 ± 17 Hz and 740 ± 11, 1098 ± 5 Hz. All detections happen when XTE J1701−407 was in its high-intensity soft state, and their single-trial significance is in the 3.1–7.5σ 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 per cent. XTE J1701−407, with a persistent luminosity close to 1 per cent 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.