Gaia pulsars and where to find them

TL;DR

This study uses Gaia DR2 data to search for companions to pulsars, confirming low observed binary fractions but suggesting many binaries remain undetected, impacting neutron-star merger rate estimates.

Contribution

It provides the first empirical constraints on the binary fraction of pulsars using Gaia data, including new potential companions and relations for identifying pulsar companions.

Findings

Confirmed low multiplicity fraction among pulsars.

Set upper limits on binary fractions for young and massive star pulsars.

Provided constraints on the galactic neutron-star merger rate.

Abstract

While the majority of massive stars have a stellar companion, most pulsars appear to be isolated. Taken at face value, this suggests that most massive binaries break apart due to strong natal kicks received in supernova explosions. However, the observed binary fraction can still be subject to strong selection effects, as monitoring of newly discovered pulsars is rarely carried out for long enough to conclusively rule out multiplicity. Here, we use the second Gaia Data Release (DR2) to search for companions to 1534 rotation-powered pulsars with positions known to better than 0.5 arcseconds. We find 22 matches to known pulsars, including one not reported elsewhere, and 8 new possible companions to young pulsars. We examine the photometric and kinematic properties of these systems and provide empirical relations for identifying Gaia sources with potential millisecond pulsar companions. Our results confirm that the observed multiplicity fraction is small. However, we show that the number of binaries below the sensitivity of Gaia and radio timing in our sample could still be significantly higher. We constrain the binary fraction of young pulsars to be $f_{\rm young}^{\rm true}\leq 5.3(8.3)\%$ under realistic(conservative) assumptions for the binary properties and current sensitivity thresholds. For massive stars ($\geq 10$ M$_{\odot}$) in particular, we find $f_{\rm OB}^{\rm true}\leq 3.7\%$ which sets a firm independent upper limit on the galactic neutron-star merger rate, $\leq 7.2\times 10^{-4}$ yr$^{-1}$. Ongoing and future projects such as the CHIME/pulsar program, MeerTime, HIRAX and ultimately the SKA, will significantly improve these constraints in the future.