We analysed Rossi X‐ray Timing Explorer (RXTE)/PCA and HEXTE data of the transient black hole binary GX 339‐4, collected over a time‐span of 8 years. We studied the properties and the behaviour of low‐frequency quasi‐periodic oscillations (QPOs) as a function of the integrated broad‐band variability and the spectral parameters during four outbursts (2002, 2004, 2007 and 2010). Most of the QPOs could be classified following the ABC classification which has been proposed before. Our results show that the ABC classification can be extended to include spectral dependencies and that the three QPO types have indeed intrinsically different properties. In terms of the relation between QPO frequency and power‐law flux, types A and C QPOs may follow the same relation, whereas the type B QPOs trace out a very different relation. Type B QPO frequencies clearly correlate with the power‐law flux and are connected to local increases of the count rate. The frequencies of all QPOs observed in the rising phase of the 2002, 2007 and 2010 outbursts correlate with the disc flux. Our results can be interpreted within the framework of the recently proposed QPO models involving Lense–Thirring precession. We suggest that types C and A QPOs might be connected and could be interpreted as being the result of the same phenomenon observed at different stages of the outburst evolution, while a different physical process produces type B QPOs.

We present a systematic analysis of the fast time variability properties of the transient black hole binary GRO J1655-40, based on the complete set of Rossi XTE observations. We demonstrate that the frequencies of the quasi periodic oscillations and of the broad band noise components and their variations match accurately the strong field general relativistic frequencies of particle motion in the close vicinity of the innermost stable circular orbit, as predicted by the relativistic precession model.We obtain high precision measurements of the black hole mass (M = (5.31 ± 0.07) M⊙, consistent with the value from optical/NIR observations) and spin (a = 0.290 ± 0.003), through the sole use of X-ray timing.

We present a systematic study of the orbital inclination effects on black-hole transients fast time-variability properties. We have considered all the black-hole binaries that have been densely monitored by the Rossi XTE satellite. We find that the amplitude of low-frequency quasi periodic oscillations (QPOs) depends on the orbital inclination. Type-C QPOs are stronger for nearly edge-on systems (high inclination), while type-B QPOs are stronger when the accretion disk is closer to face-on (low inclination). Our results also suggest that the noise associated with type-C QPOs is consistent with being stronger for low-inclination sources, while the noise associated to type-B QPOs seems inclination independent. These results are consistent with a geometric origin of the type-C QPOs -for instance arising from relativistic precession of the inner flow within a truncated disk -while the noise would correspond to intrinsic brightness variability from mass accretion rate fluctuations in the accretion flow. The opposite behavior of type-B QPOs -stronger in low inclinations sources -supports the hypothesis that type-B QPOs are related to the jet, the power of which is the most obvious measurable parameter expected to be stronger in nearly face-on sources.

We report on X‐ray observations of the black hole candidate GX 339−4 during its 2006/2007 outburst. The hardness–intensity diagram (HID) of all RXTE/Proportional Counter Array data combined shows a q‐shaped track similar to that observed in previous outbursts. The evolution through the HID suggests that in the early phase of the outburst the source underwent a sequence of state transitions, from the hard to the soft state, which is supported by our timing analysis. Broad‐band (4–200 keV) spectra, fitted with an exponentially cut‐off power law, show that the hard spectral component steepens during the transition from the hard to the soft state. The high‐energy cut‐off decreased monotonically from 120 to 60 keV during the brightening of the hard state, but again increased to 100 keV during the softening in the hard intermediate state. In the short‐lived soft intermediate state the cut‐off energy was ∼130 keV, but was no longer detected in the soft state. This is one of the first times that the high‐energy cut‐off has been followed in such detail across several state transitions. We find that in comparison to several other spectral parameters, the cut‐off energy changes more rapidly, just like the timing properties. The observed behaviour of the high‐energy cut‐off of GX 339−4 is also similar to that observed with RXTE–INTEGRAL–Swift during the 2005 outburst of GRO J1655−40. These results constitute a valuable reference to be considered when testing theoretical models for the production of the hard component in these systems.

We report on the first 180 days of RXTE observations of the outburst of the black hole candidate IGR J17091-3624. This source exhibits a broad variety of complex light curve patterns including periods of strong flares alternating with quiet intervals. Similar patterns in the X-ray light curves have been seen in the (up to now) unique black hole system GRS 1915+105. In the context of the variability classes defined by Belloni et al. (2000) for GRS 1915+105, we find that IGR J17091-3624 shows the ν, ρ, α, λ, β and µ classes as well as quiet periods which resemble the χ class, all occurring at 2-60 keV count rate levels which can be 10-50 times lower than observed in GRS 1915+105. The so-called ρ class "heartbeats" occur as fast as every few seconds and as slow as ∼100 seconds, tracing a loop in the hardness-intensity diagram which resembles that previously seen in GRS 1915+105. However, while GRS 1915+105 traverses this loop clockwise, IGR J17091-3624 does so in the opposite sense. We briefly discuss our findings in the context of the models proposed for GRS 1915+105 and find that either all models requiring near Eddington luminosities for GRS 1915+105-like variability fail, or IGR J17091-3624 lies at a distance well in excess of 20 kpc or, it harbors one of the least massive black holes known (< 3M ⊙ ).

We present the root mean square (rms)–intensity diagram for black hole transients. Using observations taken with the Rossi X‐ray Timing Explorer, we study the relation between the rms amplitude of the variability and the net count rate during the 2002, 2004 and 2007 outbursts of the black hole X‐ray binary GX 339−4. We find that the rms–flux relation previously observed during the hard state in X‐ray binaries does not hold for the other states, when different relations apply. These relations can be used as a good tracer of the different accretion regimes. We identify the hard, soft and intermediate states in the rms–intensity diagram. Transitions between the different states are seen to produce marked changes in the rms–flux relation. We find that one single component is required to explain the ∼40 per cent variability observed at low count rates, whereas no or very low variability is associated to the accretion‐disc thermal component.

Black hole and neutron star X-ray binary systems routinely show quasiperiodic oscillations (QPOs) in their X-ray flux. Despite being strong, easily measurable signals, their physical origin has long remained elusive. However, recent observational and theoretical work has greatly improved our understanding. Here, we briefly review the basic phenomenology of the different varieties of QPO in both black hole and neutron star systems before focusing mainly on low frequency QPOs in black hole systems, for which much of the recent progress has been made. We describe the detailed statistical properties of these QPOs and review the physical models proposed in the literature, with particular attention to those based on Lense-Thirring precession. This is a relativistic effect whereby a spinning massive object twists up the surrounding spacetime, inducing nodal precession in inclined orbits. We review the theory describing how an accretion flow reacts to the Lense-Thirring effect, including analytic theory and recent numerical simulations. We then describe recent observational tests that provide very strong evidence that at least a certain type of low frequency QPOs are a geometric effect, and good evidence that they are the result of precession. We discuss the possibility of the spin axis of the compact object being misaligned with the binary rotation axis for a large fraction of X-ray binaries, as is required for QPOs to be driven specifically by Lense-Thirring precession, as well as some outstanding gaps in our understanding and future opportunities provided by X-ray polarimeters and/or high throughput X-ray detectors.

Context. XSS J1227.0-4859 is a peculiar, hard X-ray source recently positionally associated to the Fermi-LAT source 1FGL J1227.9-4852/2FGL J1227.7-4853. Multi-wavelength observations have added information on this source, indicating a low-luminosity lowmass X-ray binary (LMXB), but its nature is still unclear. Aims. To progress in our understanding, we present new X-ray data from a monitoring campaign performed in 2011 with the XMM-Newton, RXTE, and Swift satellites and combine them with new gamma-ray data from the Fermi and AGILE satellites. We complement the study with simultaneous near-UV photometry from XMM-Newton and with previous UV/optical and near-IR data. Methods. We analysed the temporal characteristics in the X-rays, near-UV, and gamma rays and studied the broad-band spectral energy distribution from radio to gamma rays. Results. The X-ray history of XSS J1227 over 7 yr shows a persistent and rather stable low-luminosity (6 × 10 33 d 2 1 kpc erg s −1 ) source, with flares and dips being peculiar and permanent characteristics. The associated Fermi-LAT source 2FGL J1227.7-4853 is also stable over an overlapping period of 4.7 yr. Searches for X-ray fast pulsations down to msec give upper limits to pulse fractional amplitudes of 15 − 25% that do not rule out a fast spinning pulsar. The combined UV/optical/near-IR spectrum reveals a hot component at ∼13 kK and a cool one at ∼4.6 kK. The latter would suggest a late-type K2−K5 companion star, a distance range of 1.4−3.6 kpc, and an orbital period of 7-9 h. A near-UV variability ( 6 h) also suggests a longer orbital period than previously estimated. Conclusions. The analysis shows that the X-ray and UV/optical/near-IR emissions are more compatible with an accretion-powered compact object than with a rotational powered pulsar. The X-ray to UV bolometric luminosity ratio could be consistent with a binary hosting a neutron star, but the uncertainties in the radio data may also allow an LMXB black hole with a compact jet. In this case, it would be the first associated with a high-energy gamma-ray source.