Data-driven expectations for electromagnetic counterpart searches based on LIGO/Virgo public alerts

TL;DR

This paper analyzes how improved data analysis affects gravitational-wave event localization, impacting electromagnetic follow-up strategies, and provides simulations and public localization data for future observing runs.

Contribution

It introduces updated predictions for LIGO/Virgo/KAGRA localization accuracy based on O3 data, aiding follow-up planning for upcoming runs O4 and O5.

Findings

Increased detection sensitivity leads to more poorly localized events.

Localization uncertainties are larger than initially predicted.

Simulations help optimize follow-up strategies for electromagnetic counterparts.

Abstract

Searches for electromagnetic counterparts of gravitational-wave signals have redoubled since the first detection in 2017 of a binary neutron star merger with a gamma-ray burst, optical/infrared kilonova, and panchromatic afterglow. Yet, one LIGO/Virgo observing run later, there has not yet been a second, secure identification of an electromagnetic counterpart. This is not surprising given that the localization uncertainties of events in LIGO and Virgo's third observing run, O3, were much larger than predicted. We explain this by showing that improvements in data analysis that now allow LIGO/Virgo to detect weaker and hence more poorly localized events have increased the overall number of detections, of which well-localized, gold-plated events make up a smaller proportion overall. We present simulations of the next two LIGO/Virgo/KAGRA observing runs, O4 and O5, that are grounded in the statistics of O3 public alerts. To illustrate the significant impact that the updated predictions can have, we study the follow-up strategy for the Zwicky Transient Facility. Realistic and timely forecasting of gravitational-wave localization accuracy is paramount given the large commitments of telescope time and the need to prioritize which events are followed up. We include a data release of our simulated localizations as a public proposal planning resource for astronomers.