ESO Expanding Horizons White Paper: Electromagnetic counterparts of massive BH mergers with LISA

TL;DR

The paper outlines a strategy to maximize electromagnetic counterparts for LISA-detected massive black hole binary mergers by integrating gravitational-wave localization with rapid, multi-wavelength follow-up across optical/IR (e.g., Rubin/LSST), UV (ULTRASAT), X-ray (NewAthena/AXIS/THESEUS), and radio (SKA), complemented by spectroscopy (MOONS, WST) and high-resolution imaging (ELTs, ngVLA). It emphasizes that LISA’s localization precision evolves from $>100$ deg$^2$ weeks before merger to $\sim 0.01$ deg$^2$ at merger for $M_\text{tot}\sim 3\times 10^5\,M_\odot$ binaries at $z\sim 1$, driving a tiered observing strategy to identify true counterparts within large error boxes. The EM signatures are tied to circumbinary accretion, including periodic optical–X-ray variability and potential dimming near merger depending on gas availability, motivating time-domain survey designs and rapid follow-up. The paper argues for a proactive, theory–instrumentation collaboration to develop probabilistic frameworks for associating EM tracers with GW events and to prepare for identifying MBHB hosts and testing fundamental physics and cosmology with joint GW–EM data.

Abstract

The Laser Interferometer Space Antenna (LISA), adopted by ESA and scheduled for the second half of the next decade, will drive a new revolution in the rapidly growing field of gravitational-wave astronomy, by extending GW observations into the hiterto unexplored millihertz regime. One of the key source classes of LISA is merging massive black hole binaries in the 1e4-1e7 Msun mass range detectable to very high redshifts $z\sim 15$. MBHBs lighter than 1e6 Msun can potentially be identified during their inspiral weeks before coalescence, allowing for complementary electromagnetic observations before, during, and after the two MBHs merge. This white paper aims at defining the optimal strategy to maximize the number of detected EM counterparts of LISA MBHB events.