Pulsar-Black Hole Binaries in the Galactic Center

TL;DR

This paper proposes that pulsar-black hole binaries likely exist in the Galactic center due to dynamical interactions, predicts their properties, and encourages targeted radio searches to discover them, which would offer insights into strong gravity and stellar evolution.

Contribution

It introduces a formation mechanism for pulsar-black hole binaries in the Galactic center and predicts their observable properties, guiding future observational searches.

Findings

Estimated a few PSR-BHs should exist in the central parsec today.

Predicted properties include high eccentricity and wide orbits.

Suggested that deep radio searches could detect these binaries.

Abstract

Binaries consisting of a pulsar and a black hole (BH) are a holy grail of astrophysics, both for their significance for stellar evolution and for their potential application as probes of strong gravity. In spite of extensive surveys of our Galaxy and its system of globular clusters, no pulsar-black hole (PSR-BH) binary has been found to date. Clues as to where such systems might exist are therefore highly desirable. We show that if the central parsec around Sgr A* harbors a cluster of ~25,000 stellar BHs (as predicted by mass segregation arguments) and if it is also rich in recycled pulsar binaries (by analogy with globular clusters), then 3-body exchange interactions should produce PSR-BHs in the Galactic center. Simple estimates of the formation rate and survival time of these binaries suggest that a few PSR-BHs should be present in the central parsec today. The proposed formation mechanism makes unique predictions for the PSR-BH properties: 1) the binary would reside within ~1 pc of Sgr A*; 2) the pulsar would be recycled, with a period of ~1 to a few tens of milliseconds, and a low magnetic field B<~10^10 G; 3) the binary would have high eccentricity, e~0.8, but with a large scatter; and 4) the binary would be relatively wide, with semi-major axis a_b~0.1 - >~3 AU. The potential discovery of a PSR-BH binary therefore provides a strong motivation for deep, high-frequency radio searches for recycled pulsars toward the Galactic center.