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 (HFQPOs). 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 3σ. 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 full width at half-maximum are 67.3 ± 2.0 Hz and 4.4 ± 2.4 Hz, respectively. Their fractional rms varies between 0.4 and 2 per cent (total band detections) and between 0.5 and 3 per cent (hard band detections). As GRS 1915+105 shows large variability on time-scales longer than 1 s, we analysed the data in 16 s 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 disc 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 per cent. 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.