An early warning system for electromagnetic follow-up of gravitational-wave events

Binary neutron stars (BNSs) will spend ?10 -- 15 minutes in the band of Advanced LIGO and Virgo detectors at design sensitivity. Matched-filtering of gravitational-wave (GW) data could in principle accumulate enough signal-to-noise ratio (SNR) to identify a forthcoming event tens of seconds before the companions collide and merge. Here we report on the design and testing of an early warning gravitational-wave detection pipeline. Early warning alerts can be produced for sources that are at low enough redshift so that a large enough SNR accumulates ?10?60s before merger. We find that about 6% (respectively, 48%) of the total detectable BNS mergers will be detected 60s (10s) before the merger. About 2% of the total detectable BNS mergers will be detected before merger and localized to within 100deg2 (90% credible interval). Coordinated observing by several wide-field telescopes could capture the event seconds before or after the merger. LIGO-Virgo detectors at design sensitivity could facilitate observing at least one event at the onset of merger.