Electromagnetic follow-up of GW candidates may help confirm GW candidates that would not be confidently identified from GW observations alone. However, such follow-ups would need to deal with large position uncertainties, with areas of many tens to thousands of square degrees. This is likely to remain the situation until late in the decade. Optimizing the electromagnetic follow-up and source identification is an outstanding research topic (see, e.g., [15, 5, 67, 99, 39, 52, 59]). Triggering of focused searches in GW data by electromagnetically-detected events can also help in recovering otherwise hidden GW signals.
Networks with at least two detectors with sensitivities of the order of 200 Mpc are expected to yield detections based purely on GW data after a few years of observation, even under the most pessimistic predictions of signal rates. Sky localization will continue to be poor until a third detector reaches a sensitivity within a factor of 2 of the others and with a broad frequency bandwidth. With a four-site detector network at final design sensitivity, we may expect a significant fraction of GW signals to be localized to within a few square degrees by GW observations alone.
The purpose of this article is to provide information to the astronomy community to facilitate planning for multi-messenger astronomy with advanced GW detectors. While the scenarios described here are our best current projections, they will likely evolve as detector installation and commissioning progress. We will therefore update this article regularly.