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

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.

Figures (6)

• Figure 1: The evolution of the CGW frequency of 3C 66B at the SSB, and the associated uncertainty from SudouIguchi2003 is shown by the pink diagonal line. The dashed red line and the shaded red region show inferred frequency and their measurement uncertainty, respectively. The vertical red band represents the observing time frame of SudouIguchi2003. The blue shaded region represents the observing time frame of IguchiOkuda2010. The green shaded region represents the observing time frame of the PPTA DR3 dataset. The gray shaded region represents one frequency bin in PPTA DR3 based on $T^{-1}_\text{obs}$.
• Figure 2: Observational constraints on the chirp mass $\mathcal{M}$ of an SMBHB in 3C 66B. The histograms show a posterior for $\mathcal{M}$ based on the analysis of PPTA DR3. The dashed lines show the respective upper limits at $95\%$ credibility based on the astrophysically motivated prior $\pi(\log_{10}\mathcal{M})=\mathcal{U}(7.5,9.5)~[\log_{10}M_\odot]$. The dotted lines show the limits based on the conservative prior $\pi(\mathcal{M})=\mathcal{U}(10^{7.5},10^{9.5})~[M_\odot]$. In green, we show the results of the analysis with only the Earth term of the signal modeled. In blue, we show the results based on modeling both the Earth term and the pulsar term of the CGW. The black solid line with arbitrary density normalization represents the posterior we obtain from the 3 mm data analyzed in IguchiOkuda2010. The vertical orange line and the band correspond to the value and the uncertainty reported in IguchiOkuda2010.
• Figure 3: Observational constraints on the CGW strain amplitude, $h_0$, of an SMBHB in 3C 66B. The histograms show the posterior on $h_0$ based on the analysis of PPTA DR3. The dashed lines show the respective upper limits at $95\%$ credibility based on the astrophysically motivated prior $\pi(\log_{10}h_0)=\mathcal{U}(-17.0,-12.5)$. The dotted lines show the limits based on the conservative prior $\pi(h_0)=\mathcal{U}(10^{-17.0},10^{-12.5})$. In green, we show the results of the analysis with only the Earth term of the signal modeled. In blue, we show the results based on modeling both the Earth term and the pulsar term of the CGW. The black solid line with arbitrary density normalization represents the theoretically expected GWB contribution at the frequency of 3C 66B. The vertical orange line and the band correspond to the value and the uncertainty reported in IguchiOkuda2010.
• Figure 4: Posterior for the chirp mass $\mathcal{M}$ and frequency $f_\text{GW}$ of 3C 66B. The color map shows the number of posterior samples in $\mathcal{M}$-$f_\text{GW}$ space. The best-fit value from IguchiOkuda2010 is shown as a red star. The blue solid line shows the respective upper limit at $95\%$ credibility based on the astrophysically motivated prior $\pi(\log_{10}\mathcal{M})=\mathcal{U}(7.5,10.5)~[\log_{10}M_\odot]$. The yellow dashed line shows the limits based on the conservative prior $\pi(\mathcal{M})=\mathcal{U}(10^{7.5},10^{10.5})~[M_\odot]$.
• Figure 5: The background distribution of the $\mathcal{F}_\text{e}$ statistic, which is a chi-squared distribution with four degrees of freedom. The green vertical dashed line is the $2\mathcal{F}_\text{e}$ statistic calculated at the frequency and sky position of 3C 66B.