The two high frequency quasi periodic oscillations ( HFQPOs ) recently reported in the black hole candidate IGRÂ J17091-3624 by Altamirano and Belloni ( 2012 ) are in a 5:2 frequency ratio ( 164 Hz to 66 Hz ) .
This ratio is strongly suggestive of period doubling and nonlinear resonance analogous to phenomena known in RVÂ Tauri-type pulsating stars ( and recently discovered also in oscillations of RRÂ Lyrae-type and of BLÂ Herculis-type variables ) .
An interpretation of the frequency ratio in terms of nonlinear interactions and a comparison with the HFQPOs reported in GRS 1915+105 may imply a mass of about 6 solar masses for the black hole in IGR J17091-3624 .