Primordial black hole-star binaries via dynamical friction

TL;DR

This paper proposes a new formation channel for star-primordial black hole binaries via dynamical friction in dark matter minihalos, predicting observable X-ray binaries and gravitational wave events, and explaining recent astrophysical anomalies.

Contribution

It introduces a novel binary formation mechanism involving dynamical friction, with estimates of merger rates and implications for X-ray and gravitational wave observations.

Findings

Predicts $ ext{O}(1)$ observable short-lived X-ray binaries.

Estimates a gravitational wave event rate of $ ext{O}(0.3)$ Gpc$^{-3}$yr$^{-1}$.

Most mergers occur in low-mass galaxies.

Abstract

We study a new channel for binary system formation involving stars and stellar-mass primordial black holes (PBHs) embedded in dark matter (DM) minihalos. In this scenario, binaries form when a star passes through the DM minihalo surrounding a PBH and loses sufficient energy due to dynamical friction. The continued energy loss induced by this friction is expected to drive the resulting systems to merge rapidly. We estimate their merger rate and explore the implications for two observables: rapidly decaying X-ray binaries in the Milky Way and gravitational waves sourced by compact object mergers. We find that, for a PBH abundance $\Omega_\text{PBH}/\Omega_\text{DM} = 0.01$, this mechanism naturally produces a population of $\mathcal{O}(1)$ currently observable short-lived X-ray binaries. It also leads to a non-negligible gravitational wave event rate of $\mathcal{O}(0.3)$ Gpc$^{-3}$yr$^{-1}$, potentially involving high mass ratios and black holes in the lower or upper mass gap. Notably, most mergers arise in low-mass galaxies, making the latter rate sensitive to the low-mass end of the galaxy stellar mass function. The dynamical friction channel thus offers a plausible explanation for several unusual observations reported in recent years across both the X-ray and gravitational wave domains.