Parallel sessions

The upcoming 4th observing run (O4) of the LIGO-Virgo-Kagra gravitational-wave (GW) detector network offers the opportunity for astronomers to delve into the treasure trove of local compact binary mergers once more. Observers worldwide will resume the hunt for electromagnetic (EM) transients like kilonovae, the radioactive signature of heavy element formation, that could plausibly be associated with gravitational-wave (GW) signals. The possibility of GW-EM multi-messenger detections like those obtained for the historic binary neutron star merger GW170817 remains a tantalising prospect, with a single discovery holding the potential to greatly advance our understanding of heavy element nucleogenesis and extreme physics at nuclear densities. The anticipated threefold increase in the GW search volume compared to O3 promises an increase in the rate of triggers that correspondingly enhances the opportunity for discoveries to be made. However, it also presents challenges for the EM follow-up community on several fronts: with limited telescope time, which events should we prioritise and how could we tell? How can the still-large GW localisation regions be searched efficiently? How can unrelated transients be quickly rejected in order to reach the true EM counterparts? What do these counterparts look like? How could the numerous independent search teams collaborate to ensure that no opportunity for discovery is wasted? This session will convene the GW-EM follow-up community ahead of O4 to discuss our current understanding of kilonovae and other GW-associated transients and to share ideas on how best to solve the challenges of rapidly searching for poorly localised and fast-evolving transients. We will review our current understanding of the electromagnetic signatures expected from compact binary mergers. We will also discuss the expectations, strategies and community tools available to optimise the efforts of the GW-EM follow-up community during O4.