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

Understand

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.