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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.