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 stars1, and those of short (less than two seconds) duration by the merger of compact objects, such as two neutron stars2. A third class of events with hybrid high-energy properties was identified3, but never conclusively linked to a stellar progenitor. The lack of bright supernovae rules out typical core-collapse explosions4–6, but their distance scales prevent sensitive searches for direct signatures of a progenitor system. Only tentative evidence for a kilonova has been presented7,8. 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 1042 erg per second) kilonova possibly formed in the ejecta of a compact object merger.

Aims. Gamma-ray burst (GRB) 190829A (z = 0.0785) was detected by Fermi and Swift and also at very high energy (VHE) by the High-Energy Stereoscopic System (H.E.S.S.) telescopes. The prompt emission displayed two emission episodes separated by a quiescent gap of ∼40 s. We present the 10.4 m Gran Telescopio Canarias (GTC) observations of the afterglow of GRB 190829A and its underlying supernova. We also compare GRB 190829A to GRB 180728A, a GRB with similar behaviour, and discuss the implications on underlying physical mechanisms producing these two GRBs. Methods. We present multi-band photometric data along with spectroscopic follow-up observations taken with the 10.4 m GTC telescope. Together with the data from the prompt emission, the 10.4 m GTC data are used to understand the emission mechanisms and possible progenitor. Results. A detailed analysis of the multi-band observations of the afterglow requires the cooling frequency to pass between the optical and X-ray bands at early epochs. The afterglow then transitions to the underlying supernova (SN) 2019oyw, which dominates later on. Conclusions. Although the prompt emission temporal properties of GRB 190829A and GRB 180728A are similar, the two pulses are different in the spectral domain. We find that SN 2019oyw associated with GRB 190829A is powered by Ni decay and is a Type Ic-BL SN. The spectroscopic and photometric properties of this SN are consistent with those observed for SN 1998bw, but evolved earlier.

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