DISCOVERY OF HIGH-FREQUENCY QUASI-PERIODIC OSCILLATIONS IN THE BLACK HOLE CANDIDATE IGR J17091–3624

We report the discovery of 8.5σ high-frequency quasi-periodic oscillations (HFQPOs) at 66 Hz in the Rossi X-ray Timing Explorer data of the black hole candidate IGR J17091–3624, a system whose X-ray properties are very similar to those of microquasar GRS 1915+105. The centroid frequency of the strongest peak is ∼66 Hz, its quality factor above five, and its rms is between 4% and 10%. We found a possible additional peak at 164 Hz when selecting a subset of the data; however, at the 4.5σ level we consider this detection marginal. These QPOs have hard spectrum and are stronger in observations performed between 2011 September and October, during which IGR J17091–3624 displayed for the first time light curves that resemble those of the γ variability class in GRS 1915+105. We find that the 66 Hz QPO is also present in previous observations (4.5σ), but only when averaging ∼235 ks of relatively high count rate data. The fact that the HFQPOs frequency in IGR J17091–3624 matches surprisingly well with that seen in GRS 1915+105 raises questions on the mass scaling of QPOs frequency in these two systems. We discuss some possible interpretations; however, they all strongly depend on the distance and mass of IGR J17091–3624, both completely unconstrained today.