Discovery of high-frequency quasi-periodic oscillations in the black-hole candidate IGR J17091-3624

TL;DR

This paper reports the discovery of high-frequency quasi-periodic oscillations at 66 Hz in the black hole candidate IGR J17091-3624, revealing similarities with GRS 1915+105 and raising questions about mass scaling of QPOs.

Contribution

First detection of 66 Hz HFQPOs in IGR J17091-3624, linking its properties to GRS 1915+105 and prompting new insights into black hole system behaviors.

Findings

HFQPO at 66 Hz with high significance detected

Possible additional peak at 164 Hz, marginal detection

HFQPOs are stronger during specific light curve states

Abstract

We report the discovery of 8.5 sigma high-frequency quasi-periodic oscillations (HFQPOs) at 66 Hz in the RXTE 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 5 and its rms is between 4 and 10%. We found a possible additional peak at 164 Hz when selecting a subset of data; however, at 4.5 sigma level we consider this detection marginal. These QPOs have hard spectrum and are stronger in observations performed between September and October 2011, during which IGR J17091-3624 displayed for the first time light curves which resemble those of the gamma variability class in GRS 1915+105. We find that the 66 Hz QPO is also present in previous observations (4.5 sigma), but only when averaging ~235 ksec of relatively high count rate data. The fact that the HFQPOs frequency in IGR J17091-3624 matches surprisingly well 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.