Presentation #117.26 in the session Time-Domain Astrophysics.
Gamma-ray bursts (GRBs) are flashes of high-energy radiation arising from energetic cosmic explosions. Bursts of long (>2 s) duration are produced by the core-collapse of massive stars, those of short (< 2 s) duration by the merger of two neutron stars (NSs). Here I will present observations of the exceptionally bright GRB211211A, a long duration burst at a distance of only 346 Mpc. Our measurements indicate that its lower-energy (from ultraviolet to near-infrared) counterpart is powered by a luminous (~1042 erg s-1) kilonova formed in the ejecta of a compact binary merger. This event demonstrates that compact object mergers can power high-energy transients of over a minute long duration, and that these represent a new, although possibly rare, electromagnetic counterpart of gravitational wave radiation.