Pulsar Timing Array Constraints on the Merger Timescale of Subparsec Supermassive Black Hole Binary Candidates

TL;DR

This study constrains the merger timescale of supermassive black hole binary candidates using pulsar timing array data, finding that the average coalescence time exceeds 60,000 years, consistent with theoretical models.

Contribution

It provides new limits on SMBHB merger timescales based on PTA data, supporting the binary nature of candidates and demonstrating the effectiveness of multi-messenger approaches.

Findings

Average merger timescale > 6×10^4 years assuming GW-driven evolution.

If some SMBHBs are inactive, timescale could be > 6×10^5 years.

Results align with theoretical predictions and support SMBHB candidate validity.

Abstract

We estimate the merger timescale of spectroscopically-selected, subparsec supermassive black hole binary (SMBHB) candidates by comparing their expected contribution to the gravitational wave background (GWB) with the sensitivity of current pulsar timing array (PTA) experiments and in particular, with the latest upper limit placed by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). We find that the average timescale to coalescence of such SMBHBs is $\langle t_{\rm evol} \rangle > 6\times 10^4\,$yr, assuming that their orbital evolution in the PTA frequency band is driven by emission of gravitational waves. If some fraction of SMBHBs do not reside in spectroscopically detected active galaxies, and their incidence in active and inactive galaxies is similar, then the merger timescale could be $\sim 10$ times longer, $\langle t_{\rm evol} \rangle > 6\times 10^5\,$yr. These limits are consistent with the range of timescales predicted by theoretical models and imply that all the SMBHB candidates in our spectroscopic sample could be binaries without violating the observational constraints on the GWB. This result illustrates the power of the multi-messenger approach, facilitated by the PTAs, in providing an independent statistical test of the nature of SMBHB candidates discovered in electromagnetic searches.