We present a 2-day long Rossi X-ray Timing Explorer observation and simultaneous Swift data of the bright X-ray transient XTE J1752-223. Spectral and timing properties were stable during the observation. The energy spectrum is well described by a broken power law with a high-energy cut-off. A cold disc (∼0.3 keV) is observed when Swift/X-Ray Telescope data are considered. The fractional root mean square amplitude of the aperiodic variability (0.002-128 Hz) is 48.2 ± 0.1 per cent, and it is not energy dependent. The continuum of the power density spectrum can be fitted by using four broad-band Lorentzians. A highfrequency (∼21 Hz) component and two weak quasi-periodic oscillation-like features are also present. Time lags between soft and hard X-rays roughly follow the relation t ∝ ν −0.7 , with delays dropping from ∼0.5 (0.003 Hz) to ∼0.0015 (≥10 Hz) s. Our results are consistent with XTE J1752-223 being a black hole candidate, with all timing and spectral components very similar to those of Cyg X-1 during its canonical hard state.
XTE J1908+094 is an X-ray transient black hole candidate in the Galactic plane that was observed in outburst in 2002 and 2013. Here we present multi-frequency radio and X-ray data, including radio polarimetry, spanning the entire period of the 2013 outburst. We find that the X-ray behaviour of XTE J1908+094 traces the standard black hole hardness-intensity path, evolving from a hard state, through a soft state, before returning to a hard state and quiescence. Its radio behaviour is typical of a compact jet that becomes quenched before discrete ejecta are launched during the late stages of X-ray softening. The radio and X-ray fluxes, as well as the light curve morphologies, are consistent with those observed during the 2002 outburst of this source. The polarisation angle during the rise of the outburst infers a jet orientation in agreement with resolved observations but also displays a gradual drift, which we associate with observed changes in the structure of the discrete ejecta. We also observe an unexpected 90• rotation of the polarisation angle associated with a second component.
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.
We studied the properties of the low-frequency quasi-periodic oscillations detected in a sample of six black hole candidates (XTE J1550-564, H 1743-322, XTE J1859+226, 4U 1630, XTE J1650-500) observed by the Rossi XTE satellite. We analyzed the relation between the full width half maximum and the frequency of all the narrow peaks detected in power density spectra where a type-C QPO is observed. Our goal was to understand the nature of the modulation of the signal by comparing the properties of different harmonic peaks in the power density spectrum. We find that for the sources in our sample the width of the fundamental and of the first harmonic are compatible with a frequency modulation, while that of the sub-harmonic is independent of frequency, possibly indicating the presence of an additional modulation in amplitude. We compare our results with those obtained earlier from GRS 1915+105 and XTE J1550-564.
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