Measuring the Spins of Stellar Black Holes: A Progress Report

TL;DR

The study addresses the problem of measuring stellar-mass black-hole spins, a key factor for understanding jets, gamma-ray bursts, and gravitational-wave source populations. It advances the continuum-fitting approach by fitting the thermal X-ray continuum within the Novikov–Thorne thin-disk framework, mapping the ISCO radius to the spin parameter $a_*$ using relativistic disk models such as $kerrabb$ and $bhspec$, and extending applicability with empirical Comptonization modeling via $simpl$. The authors report spin measurements for six BH binaries, spanning from near-zero to near-extreme spins, and outline ongoing work to expand to additional systems, along with substantial methodological developments, public-release plans for software, and GRMHD validation of the NT model. They demonstrate that, within thin-disk regimes, the NT-based spin inferences are robust and discuss extensions to thicker disks, uncertainties via Monte Carlo error analysis, and the broader astrophysical implications for jets, GRBs, BH formation, and gravitational-wave astronomy. Overall, the work strengthens the reliability and scope of CF-based spin measurements, enabling more comprehensive spin catalogs and informing both theoretical models and observational programs across high-energy and gravitational-wave astrophysics.

Abstract

We use the Novikov-Thorne thin disk model to fit the thermal continuum X-ray spectra of black hole X-ray binaries, and thereby extract the dimensionless spin parameter a* = a/M of the black hole as a parameter of the fit. We summarize the results obtained to date for six systems and describe work in progress on additional systems. We also describe recent methodological advances, our current efforts to make our analysis software fully available to others, and our theoretical efforts to validate the Novikov-Thorne model.