Estimates of the black hole mass M and dimensionless spin a in the microquasar GRO J1655-40 implied by strong gravity effects related to the timing and spectral measurements are controversial , if the mass restriction determined by the dynamics related to independent optical measurements , M _ { opt } = ( 5.4 \pm 0.3 ) M _ { \odot } , are applied .
The timing measurements of twin high-frequency ( HF ) quasiperiodic oscillations ( QPOs ) with frequency ratio 3 : 2 and the simultaneously observed low-frequency ( LF ) QPO imply the spin in the range a \in ( 0.27 - 0.29 ) if models based on the frequencies of the geodesic epicyclic motion are used to fit the timing measurements , and correlated creation of the twin HF QPOs and the LF QPO at a common radius is assumed .
On the other hand , the spectral continuum method implies a \in ( 0.65 - 0.75 ) , and the Fe-line-profile method implies a \in ( 0.94 - 0.98 ) .
This controversy can be cured , if we abandon the assumption of the occurrence of the twin HF QPOs and the simultaneously observed LF QPO at a common radius .
We demonstrate that the epicyclic resonance model of the twin HF QPOs is able to predict the spin in agreement with the Fe-profile method , but no model based on the geodesic epicyclic frequencies can be in agreement with the spectral continuum method .
We also show that the non-geodesic string loop oscillation model of twin HF QPOs predicts spin a > 0.3 under the optical measurement limit on the black hole mass , in agreement with both the spectral continuum and Fe-profile methods .