Radio Continuum Studies of Ultra-Compact and Short Orbital Period X-Ray Binaries

TL;DR

This study investigates the radio emissions of ultra-compact X-ray binaries in globular clusters, analyzing their properties, formation likelihood, and the relationship between radio luminosity and orbital period, revealing correlations with cluster characteristics.

Contribution

It provides the first radio continuum counterparts for UCXBs and analyzes how globular cluster properties influence UCXB formation and radio emissions.

Findings

UCXBs are more common in compact, high-concentration GCs.

No clear correlation between radio luminosity and orbital period.

GC properties like concentration and encounter rates influence UCXB presence.

Abstract

We present the radio continuum counterparts to the enigmatic ultra-compact X-ray binaries (UCXBs); a black hole or neutron star accreting from a hydrogen-deficient white dwarf donor star, with short orbital periods ($<$ 80 minutes). For the sample of UCXBs hosted by globular clusters (GCs), we search for whether certain GC properties are more likely to enhance UCXB formation. We determine that GCs which host UCXBs are drawn from a distinct population in terms of cluster concentration, core radius and half-light radius, but are similar to other well-studied GCs in metallicity and cluster mass. In particular, UCXB-hosting GCs tend to be on average more compact, with a higher concentration than other GCs, with significantly higher encounter rates. We investigate whether a correlation exists between radio luminosity and orbital period, using new and archival observations. We determine that there is not a clear connection between the two observable quantities.