Challenges in Forming Millisecond Pulsar-Black Holes from Isolated Binaries

TL;DR

This study models the formation of millisecond pulsar-black hole binaries in isolation, concluding such systems are extremely rare and unlikely to form without dynamical interactions.

Contribution

It provides the first detailed population synthesis analysis showing the rarity and formation challenges of MSP-BH binaries in isolated environments.

Findings

NSBH binaries are very rare with a birth rate < 1 per million years.

No MSP-BH binaries undergo mass transfer after the first supernova.

Dynamical environments may be necessary for MSP-BH binary formation.

Abstract

Binaries harboring a millisecond pulsar (MSP) and a black hole (BH) are a key observing target for current and upcoming pulsar surveys. We model the formation and evolution of such binaries in isolation at solar metallicity using the next-generation binary population synthesis code POSYDON. We examine neutron star (NS)-BH binaries where the NS forms first (labeled NSBH), as the NS must be able to spin-up to MSP rotation periods before the BH forms in these systems. We find that NSBHs are very rare and have a birth rate < 1 Myr$^{-1}$ for a Milky Way-like galaxy in our typical models. The NSBH birth rate is 2-3 orders of magnitude smaller than that for NS-BHs where the BH forms first (labeled BHNS). These rates are also sensitive to model assumptions about the supernova (SN) remnant masses, natal kicks, and common-envelope efficiency. We find that 100% of NSBHs undergo a mass ratio reversal before the first SN and up to 64% of NSBHs undergo a double common envelope phase after the mass ratio reversal occurs. Most importantly, no NSBH binaries in our populations undergo a mass transfer phase, either stable or unstable, after the first SN. This implies that there is no possibility of pulsar spin-up via accretion, and thus MSP-BH binaries cannot form. Thus, dynamical environments and processes may provide the only formation channels for such MSP-BH binaries.