The Spin of the Near-Extreme Kerr Black Hole GRS 1915+105

TL;DR

This study applies continuum-fitting with advanced relativistic disk models to a rigorously selected thermal-state dataset of GRS $1915+105$ to measure black hole spin. By combining kerrbb2 with bhspec to accurately model spectral hardening and by testing multiple tail components, the authors demonstrate that the dimensionless spin parameter satisfies a robust lower bound of $a_* > 0.98$, largely independent of $M$, $D$, and $i$ at low accretion luminosities ($L/L_{\rm Edd} < 0.3$). They show that the spin estimate remains stable across tail modeling choices and is further insensitive to returning radiation, with torque effects becoming relevant only at higher luminosities. The results imply a natal origin for the extreme spin, bear on jet production and GRB-related physics, and motivate a direct observational test via VLBA parallax measurements to constrain distance and mass functions, thereby solidifying the connection between spin, jets, and gravitational-wave source modeling.

Abstract

Based on a spectral analysis of the X-ray continuum that employs a fully relativistic accretion-disk model, we conclude that the compact primary of the binary X-ray source GRS 1915+105 is a rapidly-rotating Kerr black hole. We find a lower limit on the dimensionless spin parameter of a* greater than 0.98. Our result is robust in the sense that it is independent of the details of the data analysis and insensitive to the uncertainties in the mass and distance of the black hole. Furthermore, our accretion-disk model includes an advanced treatment of spectral hardening. Our data selection relies on a rigorous and quantitative definition of the thermal state of black hole binaries, which we used to screen all of the available RXTE and ASCA data for the thermal state of GRS 1915+105. In addition, we focus on those data for which the accretion disk luminosity is less than 30% of the Eddington luminosity. We argue that these low-luminosity data are most appropriate for the thin alpha-disk model that we employ. We assume that there is zero torque at the inner edge of the disk, as is likely when the disk is thin, although we show that the presence of a significant torque does not affect our results. Our model and the model of the relativistic jets observed for this source constrain the distance and black hole mass and could thus be tested by determining a VLBA parallax distance and improving the measurement of the mass function. Finally, we comment on the significance of our results for relativistic-jet and core-collapse models, and for the detection of gravitational waves.