M31 globular cluster structures and the presence of X-ray binaries

TL;DR

This study uses Hubble Space Telescope data to analyze the structural properties of M31 globular clusters and their association with low mass X-ray binaries, highlighting the importance of collision rates and location in binary formation.

Contribution

It provides new high-resolution measurements of M31 GCs, linking structural parameters and collision rates to the likelihood of hosting X-ray binaries, supporting dynamical formation theories.

Findings

LMXB-hosting GCs have higher collision rates.

Structural differences are observed between blue and red bandpass measurements.

Probability of hosting an LMXB increases with collision rate and proximity to galaxy center.

Abstract

[Abridged] M31 has several times more globular clusters (GCs) than the Milky Way. It contains a correspondingly larger number of low mass X-ray binaries (LMXBs) associated with GCs, and can be used to investigate the GC properties which lead to X-ray binary formation. The best tracer of the spatial structure of M31 GCs is high-resolution imaging from the Hubble Space Telescope, and we have used HST data to derive structural parameters for 29 LMXB-hosting M31 GCs. These measurements are combined with structural parameters from the literature for a total of 41 (of 50 known) LMXB GCs and a comparison sample of 65 non-LMXB GCs. Structural parameters measured in blue bandpasses are found to show smaller core radii and higher concentrations than those measured in red bandpasses; this difference is enhanced in LMXB clusters and could be related to stellar population differences. Clusters with LMXBs show higher collision rates for their mass compared to those without LMXBs and collision rates estimated at the core radius show larger offsets than rates estimated at the half-light radius. These results are consistent with the dynamical formation scenario for LMXBs. A logistic regression analysis finds that, as expected, the probability of a GC hosting an LMXB increases with increasing collision rate and proximity to the galaxy center. The same analysis finds that P(LMXB) decreases with increasing GC mass at a fixed collision rate, although we caution that this could be due to sample selection effects. Metallicity is found to be a less important predictor of P(LMXB) than collision rate, mass, or distance, even though LMXB GCs have a higher metallicity on average. This may be due to the interaction of location and metallicity: a sample of M31 LMXBs with a greater range in galactocentric distance would likely contain more metal-poor GCs and make it possible to disentangle the two effects.