Precise Measurement of the Spin Parameter of the Stellar-Mass Black Hole M33 X-7

TL;DR

This study precisely measures the spin of the stellar-mass black hole in M33 X-7 using relativistic accretion disk modeling of multiple X-ray spectra, achieving consistent results across different observations with minimal uncertainty.

Contribution

It provides the first precise measurement of the black hole's spin parameter in M33 X-7 using a comprehensive relativistic disk model and multiple high-quality spectra, with uncertainties thoroughly analyzed.

Findings

Black hole spin parameter a* = 0.77 ± 0.05

Consistent spin estimates across multiple spectra

Uncertainties include model validity and spin-orbit alignment

Abstract

In prior work, {\it Chandra} and Gemini-North observations of the eclipsing X-ray binary M33 X-7 have yielded measurements of the mass of its black hole primary and the system's orbital inclination angle of unprecedented accuracy. Likewise, the distance to the binary is known to a few percent. In an analysis based on these precise results, fifteen {\it Chandra} and {\it XMM-Newton} X-ray spectra, and our fully relativistic accretion disk model, we find that the dimensionless spin parameter of the black hole primary is $a_* = 0.77 \pm 0.05$. The quoted 1-$\sigma$ error includes all sources of observational uncertainty. Four {\it Chandra} spectra of the highest quality, which were obtained over a span of several years, all lead to the same estimate of spin to within statistical errors (2%), and this estimate is confirmed by 11 spectra of lower quality. There are two remaining uncertainties: (1) the validity of the relativistic model used to analyze the observations, which is being addressed in ongoing theoretical work; and (2) our assumption that the black hole spin is approximately aligned with the angular momentum vector of the binary, which can be addressed by a future X-ray polarimetry mission.