Mass Measurements of Black Holes in X-Ray Transients: Is There a Mass Gap?

TL;DR

This study revises black hole mass estimates in X-ray transients by accounting for systematic errors, challenging the existence of a previously suggested mass gap and confirming other features of the mass distribution.

Contribution

It identifies significant systematic errors in black hole mass measurements and revises the mass distribution, negating the previously proposed mass gap.

Findings

Systematic errors can underestimate black hole masses by over ten degrees in inclination.

Revised masses eliminate the previously suggested mass gap between neutron stars and black holes.

The mass distribution peaks around 8 solar masses with a scarcity below 5 solar masses.

Abstract

We explore possible systematic errors in the mass measurements of stellar mass black holes. We find that significant errors can arise from the assumption of zero or constant emission from the accretion flow, which is commonly used when determining orbital inclination by modelling ellipsoidal variations. For A0620-00, the system with the best available data, we show that typical data sets and analysis procedures can lead to systematic underestimates of the inclination by ten degrees or more. A careful examination of the available data for the 15 other X-ray transients with low-mass donors suggests that this effect may significantly reduce the black hole mass estimates in several other cases, most notably that of GRO J0422+32. With these revisions, our analysis of the black hole mass distribution in soft X-ray transients does not suggest any "mass gap" between the low end of the distribution and the maximum theoretical neutron star mass, as has been identified in previous studies. Nevertheless, we find that the mass distribution retains other previously identified characteristics, namely a peak around 8M\odot, a paucity of sources with masses below 5M\odot, and a sharp drop-off above 10M\odot.