Possible twin kHz Quasi Periodic Oscillations in the accreting millisecond X-ray Pulsar IGR J17511-3057

We report on the aperiodic X-ray timing and color behavior of the accreting millisecond X-ray pulsar (AMXP) IGR J17511-3057, using all the pointed observations obtained with the Rossi X-ray Timing Explorer Proportional Counter Array since the source's discovery on 2009 September 12. The source can be classified as an atoll source on the basis of the color and timing characteristics. It was in the hard state during the entire outburst. In the beginning and at the end of the outburst, the source exhibited what appear to be twin kHz quasi periodic oscillations (QPOs). The separation \Delta \nu between the twin QPOs is ~ 120 Hz. Contrary to expectations for slow rotators, instead of being close to the 244.8 Hz spin frequency, it is close to half the spin frequency. However, identification of the QPOs is not certain as the source does not fit perfectly in the existing scheme of correlations of aperiodic variability frequencies seen in neutron star low mass X-ray binaries (NS LMXBs), nor can a single shift factor make it fit as has been reported for other AMXPs. These results indicate that IGR J17511-3057 is a unique source differing from other AMXPs and could play a key role in advancing our understanding of not only AMXPs, but also NS LMXBs in general.Comment: 8 pages, 8 figures, accepted for publication in ApJ (accepted 2010 December 8

A jet model for the fast IR variability of the black hole X-ray binary GX 339-4

Using the simultaneous Infra-Red (IR) and X-ray light curves obtained by Kalamkar et al., we perform a Fourier analysis of the IR/X-ray timing correlations of the black hole X-ray binary (BHB) GX 339-4. The resulting IR vs X-ray Fourier coherence and lag spectra are similar to those obtained in previous studies of GX 339-4 using optical light curves. In particular, above 1 Hz, the lag spectrum features an approximately constant IR lag of about 100 ms. We model simultaneously the radio to IR Spectral Energy Distribution (SED), the IR Power Spectral Density (PSD), and the coherence and lag spectra using the jet internal shock model ISHEM assuming that the fluctuations of the jet Lorentz factor are driven by the accretion flow. It turns out that most of the spectral and timing features, including the 100-ms lag, are remarkably well-reproduced by this model. The 100-ms time-scale is then associated with the travel time from the accretion flow to the IR emitting zone. Our exploration of the parameter space favours a jet which is at most mildly relativistic (¯ < 3), and a linear and positive relation between the jet Lorentz factor and X-ray light curve i.e. (t) − 1∝LX(t). The presence of a strong Low-Frequency Quasi-Periodic Oscillation (LFQPO) in the IR light curve could be caused by jet precession driven by Lense–Thirring precession of the jet-emitting accretion flow. Our simulations confirm that this mechanism can produce an IR LFQPO similar to that observed in GX 339-4

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

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.Comment: 10 pages, 9 figures, accepted for publication in MNRA