The origin of isolated millisecond pulsars in globular clusters

Abstract

A significant fraction of millisecond pulsars (MSPs) in globular clusters (GCs) are observed as isolated objects, despite the widely accepted scenario in which MSPs are formed through recycling in compact binary systems. The origin of these isolated objects therefore remains an open problem. In this work, we propose a combination of the Heggie--Hills law for hard/soft binaries with companion ablation as a robust indicator of the fraction of isolated MSPs, $\mathcal{F}_i$, observed in GCs. We found that $\mathcal{F}_i = \left(30.59\pm4.41/a_H + 11.20\pm4.10 \right)\,\%$, where $a_H$ is the Heggie--Hills ionization radius, and that this ionization-driven model is favored over a null hypothesis in which $\mathcal{F}_i$ is independent of $a_H$, with a Bayes factor of $BF \approx 11.6$, providing substantial evidence for a physical dependence of $\mathcal{F}_i$ on $a_H$. Our framework naturally explains the observed overabundance of isolated MSPs in NGC 5139 (aka $\omega$ Centauri) and establishes binary ionization as the primary mechanism responsible for the production of isolated MSPs in GCs.