A nearby long gamma-ray burst from a merger of compact objects

Gamma-ray bursts (GRBs) are flashes of high-energy radiation arising from energetic cosmic explosions. Bursts of long (greater than two seconds) duration are produced by the core-collapse of massive stars<SUP>1</SUP>, and those of short (less than two seconds) duration by the merger of compact objects, such as two neutron stars<SUP>2</SUP>. A third class of events with hybrid high-energy properties was identified<SUP>3</SUP>, but never conclusively linked to a stellar progenitor. The lack of bright supernovae rules out typical core-collapse explosions<SUP>4-6</SUP>, but their distance scales prevent sensitive searches for direct signatures of a progenitor system. Only tentative evidence for a kilonova has been presented<SUP>7,8</SUP>. Here we report observations of the exceptionally bright GRB 211211A, which classify it as a hybrid event and constrain its distance scale to only 346 megaparsecs. Our measurements indicate that its lower-energy (from ultraviolet to near-infrared) counterpart is powered by a luminous (approximately 10<SUP>42</SUP> erg per second) kilonova possibly formed in the ejecta of a compact object merger.