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

TL;DR

This study accurately measures the black hole mass in Nova Muscae 1991 by analyzing light curves and disk emission, revealing a mass of approximately 11 solar masses and providing insights into the system's inclination and distance.

Contribution

The paper presents the first precise estimates of the black hole mass, inclination, and distance in Nova Muscae 1991 using simultaneous photometric and spectroscopic data.

Findings

Black hole mass estimated at 11.0 solar masses

Systemic inclination determined to be 43.2 degrees

Distance to the system estimated at 4.95 kpc

Abstract

The optical counterpart of the black-hole soft X-ray transient Nova Muscae 1991 has brightened by $\Delta{V}\approx0.8$ mag since its return to quiescence 23 years ago. We present the first clear evidence that the brightening of soft X-ray transients in quiescence occurs at a nearly linear rate. This discovery, and our precise determination of the disk component of emission obtained using our $simultaneous$ photometric and spectroscopic data, have allowed us to identify and accurately model archival ellipsoidal light curves of the highest quality. The simultaneity, and the strong constraint it provides on the component of disk emission, is a key element of our work. Based on our analysis of the light curves, and our earlier measurements of the mass function and mass ratio, we have obtained for Nova Muscae 1991 the first accurate estimates of its systemic inclination $i=43.2^{+2.1}_{-2.7}$ deg, and black hole mass $M=11.0^{+2.1}_{-1.4}\ M_\odot$. Based on our determination of the radius of the secondary, we estimate the distance to be $D=4.95^{+0.69}_{-0.65}$ kpc. We discuss the implications of our work for future dynamical studies of black-hole soft X-ray transients.