Controversy of the GRO J1655-40 black hole mass and spin estimates and its possible solutions

Understand

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.