False positives in gravitational wave campaigns: the electromagnetic perspective.

The gamma-ray burst, 170817A, and kilonova, AT2017gfo, are so far the only secure electromagnetic (EM) counterparts to a gravitational wave (GW) signal (GW170817). Further associations are required to obtain a clear understanding of these compact binary mergers, including their formation and their contribution to the production of heavy elements in the Universe. With the fourth LIGO-Virgo-KAGRA observing run currently underway, the hunt is on to find further EM counterparts to GW signals. However, GW localizations are large, typically tens to hundreds of square degrees. Finding the EM counterpart is not an easy task, given that within these areas, there will be a number of IR/optical/UV transient sources that are detected serendipitously and that are not necessarily related to the GW. Understanding how the light from these false positives evolves with time is important to rapidly confirm or rule out their association with the GW trigger. In this review, I discuss the steps involved in searching for the EM counterpart of a GW event, the false positives and how they can be quickly ruled out and why false positives are of interest even though they are contaminants to the GW-EM community.This article is part of the Theo Murphy meeting issue 'Multi-messenger gravitational lensing (Part 1)'.