[Paper Review] Black Holes and Pulsar Binaries

Steinn Sigurðsson|arXiv (Cornell University)|Mar 13, 2003

Stellar, planetary, and galactic studies4 citations

TL;DR

This paper proposes that black hole–pulsar (BH-PSR) binaries form via stellar evolution and dynamical interactions in both the field and globular clusters, with field-type BH-MSP systems (formed through mass transfer and recycling) being the most likely to be observed in the near future. The key result is a prediction that one such system should be detected within the next decade due to the long lifetime and high observability of recycled pulsars in tight orbits around black holes.

ABSTRACT

Stellar mass black holes are formed in the field, and are observed in binaries. Population synthesis estimates for the formation of BH-PSR binaries suggest these may be observed, but with very large formal uncertainties in the formation rate. In globular clusters, exchanges and binary interactions present more channels for BH-PSR formation, while ejection through recoil can remove a large fraction of the initial black hole population. A BH-PSR system ought to be observed in the near future, most likely in a globular cluster.

Motivation & Objective

• To estimate the formation rate and observational prospects of black hole–pulsar binaries in the Milky Way's field and globular clusters.
• To assess the role of dynamical interactions, natal kicks, and binary evolution in shaping the population of BH-PSR systems.
• To identify the most promising observational channels for detecting BH-PSR binaries, particularly in globular clusters.
• To evaluate the impact of black hole ejection via recoil on the survival of BH-PSR systems in dense stellar environments.
• To link observable properties—such as orbital eccentricity, semi-major axis, and pulsar spin—with formation mechanisms for future detection.

Proposed method

• Stellar population synthesis models are used to estimate formation rates of BH-PSR binaries, incorporating assumptions on initial mass function, binary fraction, and natal kick distributions.
• Dynamical evolution in globular clusters is modeled via Monte Carlo simulations of three-body and binary-binary encounters, focusing on black hole ejection and exchange processes.
• The paper analyzes the recoil velocity and orbital parameters of ejected pulsar binaries, using energy and momentum conservation to constrain formation histories.
• Orbital evolution is tracked through hardening and gravitational recoil, with lifetimes estimated based on post-ejection dynamics and core relaxation times.
• Statistical predictions are derived from the number of pulsars surveyed, estimating detection rates at O(10−4) per pulsar.
• Theoretical models are calibrated to observed neutron star and pulsar populations, particularly MSPs in binary systems.

Experimental results

Research questions

• RQ1 What is the expected formation rate of black hole–pulsar binaries in the field, and how does it depend on metallicity and natal kick distributions?
• RQ2 How do dynamical interactions in globular clusters affect the survival and observable characteristics of BH-PSR systems?
• RQ3 Which formation channel—BH+PSR or BH+MSP—yields the most detectable systems, and why?
• RQ4 Can the observed properties of PSR NGC6752A be explained by a dynamical ejection event involving a black hole or black hole binary?
• RQ5 What is the likelihood of detecting a BH-PSR binary in the next decade, and which cluster environment offers the best observational prospects?

Key findings

• Field-type BH-MSP binaries, formed via mass transfer and recycling, are predicted to be observable at a rate of approximately one in 5,000 to 10,000 pulsars, making detection likely within the next decade.
• The BH+MSP channel (Type II) is more likely to yield observable systems than the BH+PSR channel (Type I), due to the longer lifetime of recycled pulsars (τ ≥ 10^9 years).
• In globular clusters, dynamical ejection via recoil can remove up to a large fraction of black holes, but surviving systems are typically massive, central, and often in binary configurations.
• PSR NGC6752A is consistent with a formation history involving ejection from the cluster core via a strong recoil event, likely with a moderately massive black hole or black hole binary, at a velocity exceeding 6σ.
• Three distinct types of BH-PSR systems may exist in globular clusters: (I) IMBH-MSP binaries formed via exchange in massive clusters, (II) standard BH-MSP binaries in intermediate-density clusters, and (III) low-mass BH-MSP systems formed via NS-NS mergers in core-collapsed clusters.
• The most promising detection scenario is GC-II: a BH-MSP binary formed via exchange in an intermediate-density cluster, which may be displaced from the core but remains observable for timescales of order 10^8 years.