# A Multimessenger Search for the Supermassive Black Hole Binary in 3C 66B with the Parkes Pulsar Timing Array

## TL;DR

This study searches for gravitational waves from a suspected supermassive black hole binary in galaxy 3C 66B using pulsar timing data, setting upper limits and proposing a combined electromagnetic and gravitational-wave analysis approach.

## Contribution

It introduces a new methodology for joint likelihood analysis of electromagnetic and gravitational-wave data for SMBHBs, and assesses the candidate in 3C 66B with updated limits.

## Key findings

- No definitive detection of the SMBHB signal.
- Upper limits on chirp mass and strain amplitude are established.
- Partially rules out some electromagnetic-based parameter space.

## Abstract

A subparsec supermassive black hole binary (SMBHB) at the center of the galaxy 3C 66B is a promising candidate for continuous gravitational-wave searches with pulsar timing arrays (PTAs). In this work, we search for such a signal in the third data release of the Parkes Pulsar Timing Array. Matching our priors to estimates of binary parameters from electromagnetic observations, we find a log Bayes factor $\ln B = - 0.0027(7)$, highlighting that the source can be neither confirmed nor ruled out. We place upper limits at $95\%$ credibility on the chirp mass $M < 6.90 \times 10^{8}\ M_{\odot}$, and on the characteristic strain amplitude $\textrm{log}_{10}(h_0)< -14.44$. This partially rules out the parameter space suggested by electromagnetic (EM) observations of 3C 66B. We also independently reproduce the calculation of the chirp mass with the 3 mm flux monitor data from the unresolved core of 3C 66B. Based on this, we outline a new methodology for constructing a joint likelihood of EM and gravitational-wave data from SMBHBs. Finally, we suggest that targeted searches may allow firmly established SMBHB candidates to be treated as standard sirens, for complementary constraints on the Universe expansion rate.