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 σ 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.

Test of the string loop oscillation model using kHz quasiperiodic oscillations in a neutron star binary

“…In the other two observations twin HF QPOs has been detected at characteristic frequencies [50] f (B)L = 740 ± 5 Hz, f (B)U = 1112 ± 17 Hz,…”

“…The results are rather unexpected and very interesting, since a very special set of frequencies has been discovered at three of the whole ansamble of the RXTE observations. In one of the three observational events a single HF QPO has been detected at a characteristic frequency [50] f (A)L = f (A)U = 1153 ± 5 Hz.…”

“…The discovery of HF QPOs at the XTE J1701-407 source was first reported in [59] where it was mentioned that it is one of the least luminous atoll sources, having the X-ray luminosity L X ∼ 0.01L Edd . Later, it has been discovered that the source is a peculiar neutron star LMXB displaying all various types of the neutron star LMXB subclasses, i.e., Z-class, bright atoll and atoll [30] Recently, a detailed analysis of the HF QPOs in the source XTE J1701-407 has been reported in [50], being based on the whole ansamble of 58 RXTE observations. The results are rather unexpected and very interesting, since a very special set of frequencies has been discovered at three of the whole ansamble of the RXTE observations.…”

“…where we use the index U for the upper and the index L for the lower of the twin frequencies observed simultaneously. The other details of the detected HF QPOs, namely the fractional rms amplitudes and the quality factors, are presented in [50]. We do not discuss them here, since we concentrate our attention on the observed frequencies only.…”

“…The radial profiles of the string loop oscillations qualitatively differ from those related to the radial and vertical oscillations of the geodesic, test particle motion in the Kerr geometry, especially there is a crossing point of the radial and vertical frequencies in the Kerr black hole spacetimes for the string loop oscillations, while for the test particle oscillations such a crossing is possible only in the Kerr naked singularity spacetimes [78,70]. This property of the radial profiles of the radial and vertical frequencies of the string loop oscillations is applied here to test the string loop oscillation model [64] for fitting the special frequency set of HF QPOs observed recently in the source XTE J1701-407 that is a LMXB system containing a neutron star [50]. For simplicity, we assume that the external geometry of the neutron star can be appropriately approximated by the Kerr geometry.…”