Mass Measurements of Black Holes in X-Ray Transients: Is There a Mass Gap? (1205.1805v1)

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.

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

This paper examines the mass distribution of black holes in soft X-ray transients (SXTs) and questions the previously suggested existence of a "mass gap" between the heaviest neutron stars and the lightest black holes. The authors, Kreidberg et al., highlight how systematic errors in mass measurements may have skewed previous analyses of black hole mass distributions.

Systematic Errors in Mass Determination

The paper begins with an investigation of potential sources of systematic error in mass measurements for black hole SXTs. Key parameters, such as mass ratio, mass function, and especially orbital inclination, critically influence the calculated mass of the black hole. Systematic errors, particularly in inclination measurement, can significantly bias black hole mass estimates. A common methodological flaw is the underestimation of inclinations due to flawed assumptions about the nonstellar light component: fluctuations in the accretion disk and hotspots can distort ellipsoidal variations in the lightcurve, mimicking lower inclinations and upwardly biasing mass estimates.

Case Study of A0620-00

The authors conduct a detailed paper on the prototypical black hole SXT A0620-00. Utilizing extensive archival data, they illustrate that typical fitting methods underestimate inclination when the ellipsoidal variability is distorted by nonstellar flux. Their refined analysis adjusted previously underestimated inclinations, significantly lowering the black hole mass estimates.

Generalization to Other SXTs

The paper applies lessons from A0620-00 to sixteen other SXTs. It reassesses inclination measurements by considering potential systematic biases across these systems. For many SXTs, commonly used approximations of nonstellar contributions were found to be significantly underestimated. This comprehensive review suggests that the observed masses of these black holes have been routinely overestimated in the literature.

Implications for the Mass Distribution

With the revised inclination and mass estimates, the paper challenges the notion of a "mass gap". Through Bayesian statistical modeling, considering both updated and traditional data, the authors find that a mass distribution including black holes with masses lower than 5 M_\odot fills what was previously thought to be a gap. Their analysis specifically highlights GRO J0422+32 as pivotal, suggesting it may house a compact object filling the lower end of the gap. However, due to uncertainties regarding inclination biases for some systems, further empirical data is warranted for validation.

Conclusion and Implications for Black Hole Formation

The recalibration of mass estimations reveals a distribution characterized by a peak around 8 M_\odot and a scarcity above 10 M_\odot. This challenges models that predict a division between neutron star and black hole masses, potentially reshaping theoretical understanding of supernova explosions and black hole formation. Different evolutionary scenarios appear to govern the formation of black holes in SXTs compared to those in high-mass X-ray binaries, demanding nuanced theoretical approaches.

Future research must continue refining mass determinations for these systems, emphasizing synchronized spectroscopic and photometric observations to correctly estimate nonstellar light contributions. The revised distribution not only contests the mass gap theory but also provides new benchmarks for theoretical models of massive star evolution.