Lighting up the nanohertz gravitational wave sky: Opportunities and challenges of multimessenger astronomy with pulsar timing array experiments

¹ Dipartimento di Fisica “G. Occhialini” Università degli Studi di Milano-Bicocca Piazza della Scienza 3, I-20126 Milano, Italy
² INFN, Sezione di Milano-Bicocca Piazza della Scienza 3, 20126 Milano, Italy
³ INAF – Osservatorio Astronomico di Brera via Brera 20 I-20121 Milano, Italy
⁴ INAF – Osservatorio Astronomico di Cagliari via della Scienza 5 09047 Selargius (CA), Italy
⁵ Como Lake Center for AstroPhysics, University of Insubria 22100 Como, Italy
⁶ Donostia International Physics Centre (DIPC) Paseo Manuel de Lardizabal 4 20018 Donostia-San Sebastian, Spain
⁷ IKERBASQUE, Basque Foundation for Science E-48013 Bilbao, Spain

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 28 March 2025
Accepted: 30 November 2025

Abstract

Pulsar timing array (PTA) experiments have the potential to unveil continuous gravitational wave (CGW) signals from individual low-redshift massive black hole binaries (MBHBs). Detecting these objects in both gravitational waves (GWs) and the electromagnetic (EM) spectrum will open a new chapter in multimessenger astronomy. We investigate the feasibility of conducting multimessenger studies by combining the CGW detections from an idealized 30 year Square Kilometer Array Mid telescope PTA and the optical data from the forthcoming Legacy Survey of Space and Time (LSST). To this end, we employed the L-Galaxies semi-analytical model applied to the Millennium simulation. We generated 200 different all-sky light cones that include galaxies, massive black holes, and MBHBs whose emission is consistently modeled based on their star formation histories and gas accretion physics. Our results predict an average of ≈33 CGW detections, with signal-to-noise ratios greater than five. The MBHBs associated with the detections are typically at z  <  0.5, with masses of ∼ 3 × 10⁹ M_⊙, mass ratios > 0.6, and eccentricities ≲0.2. In terms of EM counterparts, we find less than 15% of these systems to be connected with an active galactic nucleus detectable by LSST, while their host galaxies are easily detectable (< 23 mag) massive (M_★ > 10¹¹ M_⊙) ellipticals with typical star formation rates (10⁻¹⁵ yr⁻¹ < sSRF < 10⁻¹⁰ yr⁻¹). Although the CGW-EM counterpart association is complicated by poor sky localization (only 35% of these CGWs are localized within 100 deg²), the number of galaxy host candidates can be considerably reduced (from thousands to several tens, depending on the CGW S/N) by applying priors based on the galaxy-MBH correlations. However, picking the actual host among these candidates is highly non-trivial, as they occupy a similar region in any optical color-color diagram. Our findings highlight the considerable challenges entailed in opening the low-frequency multimessenger GW sky.