Deciphering the unusual stellar progenitor of GRB 210704A

Abstract

GRB 210704A is a burst of intermediate duration (T90 ∼ 1 − 4 s) followed by a fading afterglow and an optical excess that peaked about 7 days after the explosion. Its properties, and in particular those of the excess, do not easily fit into the well established classification scheme of GRBs as being long or short, leaving the nature of its progenitor uncertain. We present multi-wavelength observations of the GRB and its counterpart, observed up to 160 days 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.

The Progenitor and Central Engine of a Peculiar GRB 230307A

“…It suggests that the GRB 230307A seems to be with an ambiguous progenitor, either one type of compact star merger (e.g., neutron star-neutron star (NS-NS), neutron star-white dwarf (NS-WD), or neutron star-black hole (NS-BH)), which is different from other typical short GRBs, or this method at least is not suitable for applying to GRB 230307A to do the classification. Moreover, GRBs 211211A, 211227A, and 210704A are believed to be from compact star mergers (Lü et al 2022;Rastinejad et al 2022;Troja et al 2022;Yang et al 2022;Becerra et al 2023;Chang et al 2023;Ferro et al 2023). In order to compare with the two peculiar long-duration GRBs, we also calculate the values of ε for those two GRBs and find that the ε values of those two GRBs are quite similar to that of GRB 230307A.…”

“…However, the measurement of T 90 is energy and instrument dependent (Qin et al 2013), and a long-duration GRB may be disguised as a short-duration GRB due to the tip-of-iceberg effect . From an observational point of view, several peculiar GRBs break the traditional understanding of GRB progenitors, namely, that long-and short-duration GRBs are originated from mergers of two compact stars and death of massive stars, respectively, e.g., GRB 200826A (Ahumada et al 2021;Zhang et al 2021;Rossi et al 2022), GRB 060614 (Gehrels et al 2006;Yang et al 2015), GRB 210704A (Becerra et al 2023), GRB 211211A (Rastinejad et al 2022;Troja et al 2022;Yang et al 2022;Chang et al 2023;Gompertz et al 2023), and GRB 211227A (Lü et al 2022;Ferro et al 2023). Lü et al (2010) proposed a new phenomenological classification method of GRBs by introducing a new parameter e = g E E p z ,iso,52 , ,2 5 3 , where E γ,iso and E p,z are the isotropic gamma-ray energy and the cosmic rest-frame spectral peak energy, respectively.…”

Identification of Extended Emission Gamma-Ray Burst Candidates Using Machine Learning

“…The SGRBs' gamma-ray emission is harder and dimmer, and they are typically seen in galaxies with low or absent star formation, while LGRBs have a softer spectrum, a larger total luminosity, and are typically seen in star-forming galaxies (Gehrels & Razzaque 2013). That said, there is a certain overlap between the two classes, which makes it difficult to separate them, especially in GRBs lasting more than 2 s Troja et al 2022;Becerra et al 2023).…”