Constraining the coalescence rate of supermassive black-hole binaries using pulsar timing

TL;DR

This paper uses pulsar timing data to constrain the rate of supermassive black-hole mergers, showing current bounds are close to theoretical predictions and future improvements could enable direct detection of individual SMBH binaries.

Contribution

It provides the first direct constraints on SMBH binary coalescence rates from pulsar timing and discusses future prospects with improved timing precision.

Findings

Current bounds are just above theoretical predictions for redshifts below 10.

Future timing improvements could exclude some SMBH merger models.

Achieving 10ns timing accuracy could enable detection of individual SMBH binaries.

Abstract

Pulsar timing observations are used to place constraints on the rate of coalescence of supermassive black-hole (SMBH) binaries as a function of mass and redshift. In contrast to the indirect constraints obtained from other techniques, pulsar timing observations provide a direct constraint on the black-hole merger rate. This is possible since pulsar timing is sensitive to the gravitational waves (GWs) emitted by these sources in the final stages of their evolution. We find that upper bounds calculated from the recently published Parkes Pulsar Timing Array data are just above theoretical predictions for redshifts below 10. In the future, with improved timing precision and longer data spans, we show that a non-detection of GWs will rule out some of the available parameter space in a particular class of SMBH binary merger models. We also show that if we can time a set of pulsars to 10ns timing accuracy, for example, using the proposed Square Kilometre Array, it should be possible to detect one or more individual SMBH binary systems.