Gigaelectronvolt emission from a compact binary merger

An energetic γ-ray burst (GRB), GRB 211211A, was observed on 11 December 2021^1,2. Despite its long duration, typically associated with bursts produced by the collapse of massive stars, the observation of an optical-infrared kilonova points to a compact binary merger origin³. Here we report observations of a significant (more than five sigma) transient-like emission in the high-energy γ-rays of GRB 211211A (more than 0.1 gigaelectronvolts) starting 10³ seconds after the burst. After an initial phase with a roughly constant flux (about 5 × 10⁻¹⁰ erg per second per square centimetre) lasting about 2 × 10⁴ seconds, the flux started decreasing and soon went undetected. Our detailed modelling of public and dedicated multi-wavelength observations demonstrates that gigaelectronvolt emission from GRB 211211A is in excess with respect to the flux predicted by the state-of-the-art afterglow model at such late time. We explore the possibility that the gigaelectronvolt excess is inverse Compton emission owing to the interaction of a late-time, low-power jet with an external source of photons, and find that kilonova emission can provide the seed photons. Our results open perspectives for observing binary neutron star mergers.