A kilonova following a long-duration gamma-ray burst at 350 Mpc

Gamma-ray bursts (GRBs) are divided into two populations<SUP>1,2</SUP>; long GRBs that derive from the core collapse of massive stars (for example, ref. <SUP>3</SUP>) and short GRBs that form in the merger of two compact objects<SUP>4,5</SUP>. Although it is common to divide the two populations at a gamma-ray duration of 2 s, classification based on duration does not always map to the progenitor. Notably, GRBs with short (≲2 s) spikes of prompt gamma-ray emission followed by prolonged, spectrally softer extended emission (EE-SGRBs) have been suggested to arise from compact object mergers<SUP>6-8</SUP>. Compact object mergers are of great astrophysical importance as the only confirmed site of rapid neutron capture (r-process) nucleosynthesis, observed in the form of so-called kilonovae<SUP>9-14</SUP>. Here we report the discovery of a possible kilonova associated with the nearby (350 Mpc), minute-duration GRB 211211A. The kilonova implies that the progenitor is a compact object merger, suggesting that GRBs with long, complex light curves can be spawned from merger events. The kilonova of GRB 211211A has a similar luminosity, duration and colour to that which accompanied the gravitational wave (GW)-detected binary neutron star (BNS) merger GW170817 (ref. <SUP>4</SUP>). Further searches for GW signals coincident with long GRBs are a promising route for future multi-messenger astronomy.