The MAVERIC Survey: Dynamical Origin of Radio Sources in Galactic Globular Clusters

Abstract

We investigate potential correlations between radio source counts (after background corrections) of 22 Galactic globular clusters (GCs) from the MAVERIC survey, and stellar encounter rates ($\Gamma$) and masses ($M$) of the GCs. Applying a radio luminosity limit of $L_\mathrm{lim}=5.0\times 10^{27}~\mathrm{erg~s^{-1}}$, we take a census of radio sources in the core and those within the half-light radius. By following a maximum likelihood method and adopting a simplified linear model, we find an unambiguous dependence of core radio source counts on $\Gamma$ and/or $M$ at 90% confidence, but no clear dependence of source counts within the half-light radius on either $\Gamma$ or $M$. Five of the identified radio sources in cores above our adopted limit are millisecond pulsars or neutron star X-ray binaries (XRBs), the dependence of which on $\Gamma$ is well-known, but another is a published black hole (BH) XRB candidate, and ten others are not identified. Accounting for the verified cluster members increases the significance of correlation with $M$ and/or $\Gamma$ (to 99% confidence), for fits to core and half-light region source counts, while excluding a dependence on $\Gamma$ alone at 90% (core) and 68% (half-light) confidence. This is consistent with published dynamical simulations of GC BH interactions that argue $\Gamma$ will be a poor predictor of the distribution of accreting BHs in GCs. Future multiwavelength follow-up to verify cluster membership will enable stronger constraints on the dependence of radio source classes on cluster properties, promising a new view on the dynamics of BHs in GCs.