Controversy of the GRO J1655-40 black hole mass and spin estimates and   its possible solutions
  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, $M_{\rm opt}=(5.4\pm0.3) M_{\odot}$, 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. 
Zden\v{e}k Stuchl\'ik, Martin Kolo\v{s}
astro-ph.HE