Fast variability from black-hole binaries

Currently available information on fast variability of the X-ray emission from accreting collapsed objects constitutes a complex phenomenology which is difficult to interpret. We review the current observational standpoint for black-hole binaries and survey models that have been proposed to interpret it. Despite the complex structure of the accretion flow, key observational diagnostics have been identified which can provide direct access to the dynamics of matter motions in the close vicinity of black holes and thus to the some of fundamental properties of curved spacetimes, where strong-field general relativistic effects can be observed.

Discovery of a 34 Hz Quasi-Periodic Oscillation in the X-ray emission of GRS 1915+105

We report the discovery in the Rossi X-Ray Timing Explorer data of GRS 1915+105 of a second quasi-periodic oscillation at 34 Hz, simultaneous with that observed at 68 Hz in the same observation. The data corresponded to those observations from 2003 where the 68-Hz oscillation was very strong. The significance of the detection is 4.2 sigma. These observations correspond to a very specific position in the colour-colour diagram for GRS 1915+105, corresponding to a harder spectrum compared to those where a 41 Hz oscillation was discovered. We discuss the possible implications of the new pair of frequencies comparing them with the existing theoretical models.

High-Frequency Quasi-Periodic Oscillations from GRS 1915+105

We report the results of a systematic timing analysis of all archival Rossi X-Ray Timing Explorer (RXTE) observations of the bright black-hole binary GRS 1915+105 in order to detect high-frequency quasi-periodic oscillations (HFQPO). We produced power-density spectra in two energy bands and limited the analysis to the frequency range 30-1000 Hz. We found 51 peaks with a single trial significance larger than 3 sigma. As all but three have centroid frequencies that are distributed between 63 and 71 Hz, we consider most of them significant regardless of the number of trials involved. The average centroid frequency and FWHM are 67.3 +/- 2.0 Hz and 4.4 +/- 2.4 Hz respectively. Their fractional rms varies between 0.4% and 2% (total band detections) and between 0.5% and 3% (hard ban detections). As GRS 1915+105 shows large variability on time scales longer than 1s, we analysed the data in 16s intervals and found that the detections are limited to a specific region in the colour-colour diagram, corresponding to state B of the source, when the energy spectrum is dominated by a bright accretion disk component. However, the rms spectrum of the HFQPO is very hard and does not show a flattening up to 40 keV, where the fractional rms reaches 11%. We discuss our findings in terms of current proposed models and compare them with the results on other black-hole binaries and neutron-star binaries.

LOFT: the Large Observatory For X-ray Timing

LOFT, the large observatory for X-ray timing, is a new mission concept competing with other four candidates for a launch opportunity in 2022-2024. LOFT will be performing high-time resolution X-ray observations of compact objects, combining for the first time an unprecedented large collecting area for X-ray photons and a spectral resolution approaching that of CCD-based X-ray instruments (down to 200 eV FWHM at 6 keV). The operating energy range is 2-80 keV. The main science goals of LOFT are the measurement of the neutron stars equation of states and the test of General Relativity in the strong field regime. The breakthrough capabilities of the instruments on-board LOFT will permit to open also new discovery windows for a wide range of Galactic and extragalactic X-ray sources. In this contribution, we provide a general description of the mission concept and summarize its main scientific capabilities.

High-Frequency Quasi-Periodic Oscillations in black-hole binaries

We present the results of the analysis of a large database of X-ray observations of 22 galactic black-hole transients with the Rossi X-Ray timing explorer throughout its operative life for a total exposure time of ~12 Ms. We excluded persistent systems and the peculiar source GRS 1915+105, as well as the most recently discovered sources. The semi-automatic homogeneous analysis was aimed at the detection of high-frequency (100-1000 Hz) quasi-periodic oscillations (QPO), of which several cases were previously reported in the literature. After taking into account the number of independent trials, we obtained 11 detections from two sources only: XTE J1550-564 and GRO J1655-40. For the former, the detected frequencies are clustered around 180 Hz and 280 Hz, as previously found. For the latter, the previously-reported dichotomy 300-450 Hz is found to be less sharp. We discuss our results in comparison with kHz QPO in neutron-star X-ray binaries and the prospects for future timing X-ray missions.

Black hole transients

Sixteen years of observations of black hole transients with the Rossi X-ray Timing Explorer, complemented by other X-ray observatories and ground-based optical/infrared/radio telescopes have given us a clear view of the complex phenomenology associated with their bright outbursts. This has led to the definition of a small number of spectral/timing states which are separated by marked transitions in observables. The association of these states and their transitions to changes in the radio emission from relativistic radio jets completes the picture and have led to the study of the connection between accretion and ejection. A good number of fundamental questions are still unanswered, but the existing picture provides a good framework on which to base theoretical studies. We discuss the current observational standpoint, with emphasis onto the spectral and timing evolution during outbursts, as well as the prospects for future missions such as ASTROSAT (2012) and LOFT (>2020 if selected).