The multimessenger view of Pulsar Timing Array black holes with the Horizon-AGN simulation

TL;DR

This work uses the Horizon-AGN cosmological simulation to construct realistic MBHB populations and quantify their GWB contribution in the PTA band, while also identifying potential CW sources and their electromagnetic counterparts. The authors develop both analytic and post-processed dynamical frameworks to evolve MBHBs (circular and eccentric) and generate 2000 universe realizations to capture cosmic variance, comparing a Gaussian ensemble view with a population-based approach. They demonstrate that, at PTA frequencies, the background is dominated by hundreds to thousands of binaries with $\mathcal{M}_c\sim10^{8.5}-10^{9.5}\,M_\odot$ at $z\sim0.05-1$, and that eccentricity distributes GW power into harmonics, bending the spectrum at $f_{\rm obs}\lesssim10$ nHz. The study also shows that CW detectability is modest for current-generation PTAs but more favorable for SKA-like sensitivities, and that EM signatures are often outshined by host galaxies except in radio and X-ray bands, motivating targeted multi-messenger searches and highlighting potential biases in PTA spectral inferences if finite-duration and leakage effects are not properly accounted for.

Abstract

We use the Horizon-AGN cosmological simulation to study the properties of supermassive black hole binaries (MBHBs) contributing most to the gravitational wave background (GWB) signal expected in the pulsar timing array (PTA) band. We develop a pipeline to generate realistic populations of MBHBs, allowing us to estimate both the characteristic strain and GWB time series observable by PTA experiments. We identify potential continuous wave (CW) candidates standing above the background noise, using toy PTA sensitivities representing the current EPTA and future SKA. We estimate the probability of detecting at least one CW with signal-to-noise ratio $>3$ to be $4\%$ ($20\%$) for EPTA (SKA)-like sensitivities, assuming a 10-year baseline. We find the GWB to be dominated by hundreds to thousands of binaries at redshifts in the range $0.05-1$, with chirp masses of $10^{8.5}-10^{9.5}\, M_\odot$, hosted mainly in quiescent massive galaxies residing in halos of mass $\sim 10^{13}\, M_\odot$. CW candidates have larger masses, lower redshifts and are found in even more massive halos, typical of galaxy groups and clusters. The majority of these systems would appear as AGN rather than quasars, because of their low Eddington ratios. Nevertheless, CW candidates with $f_{\rm Edd}>10^{-3}$ can still outshine their hosts, particularly in radio and X-ray bands, suggesting them as the most promising route for identification. Our findings imply that optical and near-infrared searches based on light curve variability are challenging and biased toward more luminous systems. Finally, we highlight important caveats in the common method used to compare PTA observations with theoretical models. We find that GWB spectral inferences used by PTAs could be biased toward shallower slopes and higher amplitudes at $f=1/\rm yr$, thereby reducing the apparent tension between astrophysical expectations and PTA observations.

Figures (16)

• Figure 1: Examples of the residence time (bottom panel), $\Delta \tau_{\rm c}$, per $\log_{10}$ orbital frequency bin are shown for three binaries from the $\hbox{\sc \small Horizon-AGN}$ catalog, with chirp masses of $10^7$, $10^8$, and $10^9 \,{\rm M_\odot}$. We also show their respective eccentricity evolution in the top panel. Eccentric systems are more efficient in dissipating the energy through GWs and this can be seen in the residence time (low panel) as compare to (nearly) circular binaries. The evolution of binaries at high frequencies (starting from a few nHz) is determined by the gravitational radiation which is more efficient in energy dissipation for heavy and eccentric binaries. We also observe a fast circularization of eccentric systems in the top panel.
• Figure 2: 68% (dark) and 90% (light) confidence regions for the characteristic strain $h_{\rm c}$ are shown for 2 000 universe realizations drawn from the $\hbox{\sc \small Horizon-AGN}$ MBHB population, assuming either circular orbits (blue) or non-circular orbits (orange). We also plot their respective deterministic mean expectation value in dotted and dashed line. For the circular population, we overlay as a dashed-dotted line the average spectrum, assuming the binary dynamics are driven by GW emission only. The inclusion of eccentricity introduces a significant signal reduction at frequency lower than $10\,\rm nHz$, due to the reduction of the MBHBs residence time at low orbital frequency.
• Figure 3: Median and 68% confidence interval of the differential contribution to the total characteristic strain signal, based on 2 000 universe realizations, shown across MBHB redshift bins for both circular and eccentric populations. We compare the results at two observer frequencies: 3.1 nHz (left panel) and 27.6 nHz (right panel). The vertical lines represent the median redshift values at which the cumulative contribution (integrating from low and high redshifts, respectively) reach 5% of the total strain signal for the circular (solid line) and eccentric (dashed line) populations.
• Figure 4: Median and 68% confidence interval of the differential contribution to the total characteristic strain signal, based on 2 000 universe realizations, shown across MBHB $\log_{10}$-chirp mass bins for both circular and eccentric populations. We compare the results at two observer frequencies: 3.1 nHz (left panel) and 27.6 nHz (right panel). Vertical lines represent the median chirp mass value at which the cumulative contribution (integrating from lower masses) reaches 10% of the total strain signal for the circular (solid line) and eccentric (dashed line) populations.
• Figure 5: Comparison of the effective number of binaries in terms of $N_{75}$ and $N_{\rm eff}$ contributing to the GWB at each observed frequency bin of a PTA with an observing duration of 10.3 years as for 2023AA...678A..50E. At each observer frequency, the two extreme horizontal lines represent the 5th and 95th quartiles of the $N_{75}$ distribution, derived from 2 000 universe realizations. The edges and the central line of the box represent the 25th, 50th, and 75th quartiles, respectively. For $N_{\rm eff}$, we only show the median of the distribution at each observer frequency with blue dots for the circular population and orange crosses for the eccentric population.