A Kilonova Following a Long-Duration Gamma-Ray Burst at 350 Mpc

Abstract Gamma-ray bursts (GRBs) are divided into two populations [1, 2]; long GRBs that derive from the core-collapse of massive stars [e.g., 3] and short GRBs that form in the merger of two compact objects [4]. While it is common to divide the two populations at a γ-ray duration of two seconds, classification based on duration does not always cleanly map to the progenitor. This is notable in the form of GRBs with bright, GRB 211211A 3 short (≲ 2 seconds) spikes of prompt γ-ray emission followed by more prolonged, spectrally-softer extended emission (EE-SGRBs), which have been suggested to arise from compact object mergers [5–7]. 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 [8–13]. Here, we report the discovery of a kilonova associated with a nearby (350 Mpc) minute-duration GRB, confirming that the progenitor is a compact object merger. This kilonova has a similar luminosity, duration and color to AT 2017gfo, the kilonova found in association with the gravitational wave (GW)-detected binary neutron star (BNS) merger GW170817 [4]. We find that the merger ejected ≈ 0.04M⊙ of r-process-rich material, and is consistent with the merger of two neutron stars (NSs) with masses close to the canonical 1.4M⊙. This discovery implies that GRBs with long, complex light curves can be spawned from compact object merger events and that a population of kilonovae following GRBs with durations ≫ 2 s should be accounted for in calculations of the NS merger r-process contribution. At 350 Mpc, the current network of GW interferometers at design sensitivity would have detected the merger precipitating GRB 211211A, had it been operating at the time of the event. Further searches for GW signals coincident with long GRBs are a promising route for future multi-messenger astronomy.