The Spin of The Black Hole in the X-ray Binary Nova Muscae 1991

TL;DR

This study measures the spin of the black hole in Nova Muscae 1991 using X-ray spectra and the continuum-fitting method, providing a more accurate estimate that challenges previous claims about its spin direction and relation to jet power.

Contribution

It presents the first precise spin measurement of the Nova Muscae 1991 black hole using high-quality X-ray spectra and confirms the black hole's prograde spin, contradicting earlier retrograde claims.

Findings

Black hole spin parameter a/M = 0.63 (+0.16, -0.19)

Spin is prograde, not retrograde

Challenges previous predictions relating spin to jet power

Abstract

The bright soft X-ray transient Nova Muscae 1991 was intensively observed during its entire 8-month outburst using the Large Area Counter (LAC) onboard the Ginga satellite. Recently, we obtained accurate estimates of the mass of the black hole primary, the orbital inclination angle of the system, and the distance. Using these crucial input data and Ginga X-ray spectra, we have measured the spin of the black hole using the continuum-fitting method. For four X-ray spectra of extraordinary quality we have determined the dimensionless spin parameter of the black hole to be a/M = 0.63 (-0.19, +0.16) (1 sigma confidence level), a result that we confirm using eleven additional spectra of lower quality. Our spin estimate challenges two published results: It is somewhat higher than the value predicted by a proposed relationship between jet power and spin; and we find that the spin of the black hole is decidedly prograde, not retrograde as has been claimed.