What is the Most Promising Electromagnetic Counterpart of a Neutron Star Binary Merger?

TL;DR

This paper evaluates various electromagnetic counterparts to neutron star mergers, concluding that short gamma-ray bursts are best for confirming events, while kilonovae are more suitable for localizing and studying large samples.

Contribution

It provides a comparative analysis of EM counterparts, recommending strategies for detection and localization to maximize scientific returns from GW events.

Findings

SGRBs are most useful for confirming GW events.

Kilonovae are preferred for localizing and obtaining redshifts.

Radio afterglows are promising only in specific energetic cases.

Abstract

The final inspiral of double neutron star and neutron star-black hole binaries are likely to be detected by advanced networks of ground-based gravitational wave (GW) interferometers. Maximizing the science returns from such a discovery will require the identification and localization of an electromagnetic (EM) counterpart. Here we critically evaluate and compare several possible counterparts, including short-duration gamma-ray bursts (SGRBs), "orphan" optical and radio afterglows, and ~day-long optical transients powered by the radioactive decay of heavy nuclei synthesized in the merger ejecta ("kilonovae"). We assess the promise of each counterpart in terms of four "Cardinal Virtues": detectability, high fraction, identifiability, and positional accuracy. Taking into account the search strategy for typical error regions of ~10s degs sq., we conclude that SGRBs are the most useful to confirm the cosmic origin of a few GW events, and to test the association with NS mergers. However, for the more ambitious goal of localizing and obtaining redshifts for a large sample of GW events, kilonovae are instead preferred. Off-axis optical afterglows will be detectable for at most ~10% of all events, while radio afterglows are promising only for the unique combination of energetic relativistic ejecta in a high density medium, and even then will require hundreds of hours of EVLA time per event. Our main recommendations are:(i) an all-sky gamma-ray satellite is essential for temporal coincidence detections, and for GW searches of gamma-ray triggered events; (ii) LSST should adopt a 1-day cadence follow-up strategy, ideally with ~0.5 hr per pointing to cover GW error regions (the standard 4-day cadence and depth will severely limit the probability of a unique identification); and (iii) radio searches should only focus on the relativistic case, which requires observations for a few months.