Deciphering the unusual stellar progenitor of GRB 210704A

GRB 210704A is a burst of intermediate duration (T-90 similar to 1-4 s) followed by a fading afterglow and an optical excess that peaked about 7 d after the explosion. Its properties, and in particular those of the excess, do not easily fit into the well-established classification scheme of gamma-ray bursts (GRBs) as being long or short, leaving the nature of its progenitor uncertain. We present multiwavelength observations of the GRB and its counterpart, observed up to 160 d after the burst. In order to decipher the nature of the progenitor system, we present a detailed analysis of the GRB high-energy properties (duration, spectral lag, and Amati correlation), its environment, and late-time optical excess. We discuss three possible scenarios: a neutron star merger, a collapsing massive star, and an atypical explosion possibly hosted in a cluster of galaxies. We find that traditional kilonova and supernova models do not match well the properties of the optical excess, leaving us with the intriguing suggestion that this event was an exotic high-energy merger.

Automated summary

GRB 210704A is an intermediate duration burst that shows an optical excess. The properties of the burst do not fit well into the established classification scheme of gamma-ray bursts, making the nature of its progenitor uncertain. Multiwavelength observations and detailed analysis suggest three possible scenarios: a neutron star merger, a collapsing massive star, or an atypical explosion possibly hosted in a cluster of galaxies. The optical excess does not match well with traditional models, suggesting an exotic high-energy merger.