The observed mass distribution of Galactic black hole LMXBs is biased against massive black holes

TL;DR

This paper investigates how observational biases in the spatial distribution of Galactic black hole low-mass X-ray binaries (LMXBs) lead to an underrepresentation of the most massive black holes in current samples, highlighting the role of selection effects.

Contribution

It demonstrates that current observational samples of BH LMXBs are biased against massive black holes due to their distribution and faintness caused by interstellar absorption.

Findings

Most BH LMXBs are located far from the Galactic Plane.

Confirmed BH LMXBs are farther from the Galactic Center than candidates.

Selection effects bias the observed sample against the most massive BHs.

Abstract

The discovery of gravitational wave radiation from merging black holes (BHs) also uncovered BHs with masses in the range of ~20-160 Msun. In contrast, the most massive Galactic stellar-mass BH currently known has a mass ~21 Msun. While low-mass X-ray binaries (LMXBs) will never independently evolve into a binary BH system, and binary evolution effects can play an important role explaining the different BH masses found through studies of X-ray binaries and gravitational wave events, (electromagnetic) selection effects may also play a role in this discrepancy. Assuming BH LMXBs originate in the Galactic Plane, we show that the spatial distribution of the current sample of confirmed and candidate BH LMXBs are both biased to sources that lie at a large distance from the Plane. Specifically, most of the confirmed and candidate BH LMXBs are found at a Galactic height larger than 3 times the scale height for massive star formation. In addition, the confirmed BH LMXBs are found at larger distances to the Galactic Center than the candidate BH LMXBs. Interstellar absorption makes candidate BH LMXBs in the Plane and those in the Bulge too faint for a dynamical mass measurement using current instrumentation. Given the observed and theoretical evidence for BH natal and/or Blaauw kicks, their relation with BH mass and binary orbital period, and the relation between outburst recurrence time and BH mass, the observational selection effects imply that the current sample of confirmed BH LMXBs is biased against the most massive BHs.