The Era of Binary Supermassive Black Holes: Coordination of Nanohertz-Frequency Gravitational-Wave Follow-up

TL;DR

The study addresses how to coordinate multi-messenger follow-up for nanohertz-frequency gravitational waves from binary supermassive black holes detected by pulsar timing arrays, anchored by the observable GW frequency $f_{gw}$, sky localization, and distance $D_L$. It gathers a cross-disciplinary approach to synthesize EM signatures, archival resources, catalogs, and cyber-infrastructure into a practical roadmap from an initial CW hint to a confident MM identification. Key contributions include clarifying EM-signature uncertainties, emphasizing the role of large galaxy catalogs and shared candidate lists, outlining multi-band GW synergies with LISA, and proposing centralized databases and contribution-tracking to sustain collaboration. The significance lies in establishing a concrete framework and community-driven infrastructure to enable robust, long-term multi-messenger MBHB discoveries in the PTA era.

Abstract

Here we summarize discussions and conclusions from the conference ``The Era of Binary Supermassive Black Holes: Coordination of Nanohertz-Frequency Gravitational-Wave Follow-up,'' held at the Aspen Center for Physics from February 2-7, 2025. The meeting facilitated a crucial knowledge exchange between electromagnetic and gravitational-wave theorists, observers, and cyber-infrastructure experts. The central goal was to guide the development of multi-messenger follow-up strategies for binary supermassive black hole detections by pulsar timing arrays. To build a common basis of understanding for the broader scientific community, this summary outlines the main considerations and recommendations from the meeting, summarizes the knowledge gaps identified, and ends with a potential roadmap to catalyze discussion about the search for electromagnetic counterparts to massive black hole binaries detected by pulsar timing arrays.

Figures (7)

• Figure 1: Talks by Witt and Petrov highlighted that in a PTA-based CW detection, mass and distance will be covariant until high signal-to-noise is reached. The above plot by P. Petrov is an illustrative example based on simulations described in petrov+24. Shading represents the density of posterior samples in the parameter space. The colored contours represent the 68% and 95% credible intervals for two simulated MBHB CW detections. Scatter points outside the contours are posterior samples that fall outside of the 95% credible interval. Both simulations used $\mathcal{M}=10^9~{\rm M_\odot}$ and $f_{\rm gw}=20$ nHz in an "IPTA Data Release 3"-like 116-pulsar PTA. The distance of the source was varied to induce a CW with a theoretical signal-to-noise ratio of 8 (blue, corresponding to 303 Mpc) and 16 (orange, 151.5 Mpc).
• Figure 2: A schematic representing regimes of gas disk (thermo)dynamics around a circular, $q=0.3$ MBHB of rest-frame orbital period $P_{\rm rest}$ and total mass $M$, based on physical arguments outlined in the talk by D'Orazio. Binaries within the regime between the "hard binary" line (below which MBHB orbital energy exceeds stellar dynamical energy) and the "disk decoupling" line (below which the binary's GW evolution may drive binary evolution faster than the disk evolution), can suffer significant interactions with a a circumbinary disk (CBD). Hot accretion flows ("Hot AF") will be generated when shearing and internal friction dominates over disk cooling mechanisms. Thus, PTA binaries might exist in a transition region of mixed thin standard accretion, self-gravitating, or hot, puffy disk flows. Lines of constant gravitational-wave inspiral timescale ($T_{\rm GW}$) and orbital semi-major axis ($a$, assuming Keplerian dynamics) are shown for convenience.
• Figure 3: An R.A.-Dec. sky map showing the positions of the WISE x SuperCOSMOS photometric redshift catalog (blue), and of the sixteenth Data Release of the Sloan Digital Sky Survey Quasar Catalog (SDSS DR16Q, red). These surveys, and 2MASS, have already obtained spectroscopic and/or photometric redshifts for a significant portion of the targets likely to be of interest to PTAs, however may be missing the most distant possible hosts veronesi, and larger errors in photometric redshift measurements may potentially lead to the erroneous omisson of a galaxy's inclusion in candidate host catalogs. Regions within a few degrees of the Galactic plane are incomplete in archives. Figure by N. Veronesi.
• Figure 4: Large-field-of-view missions this decade (augmented based on summary talk by S. Gezari). As of February 2025, the eRosita X-ray mission was paused and AXIS has not been approved, leaving a notable absence of sensitive, wide-field X-ray sensitivity.
• Figure 5: A sketch of the flow of information between the PTA community and MMA communities, as described in §\ref{['sec:cyberi']}. This figure was developed collaboratively by the Thursday evening communications discussion group.