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

TL;DR

This paper examines the conflicting estimates of the black hole mass and spin in GRO J1655-40 from different observational methods, proposing that abandoning the assumption of simultaneous twin HF and LF QPOs can reconcile these differences.

Contribution

It demonstrates that the epicyclic resonance model aligns with Fe-profile spin estimates, and introduces a non-geodesic string loop oscillation model that agrees with optical mass constraints and other methods.

Findings

Epicyclic resonance model predicts spin consistent with Fe-profile method.

String loop oscillation model predicts spin > 0.3, matching optical and spectral methods.

Abandoning the assumption of common radius for twin HF and LF QPOs resolves the controversy.

Abstract

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.