The case for a minute-long merger-driven gamma-ray burst from fast-cooling synchrotron emission

For decades, gamma-ray bursts (GRBs) have been broadly divided into long- and short-duration bursts, lasting more or less than 2 s, respectively. However, this dichotomy does not perfectly map to the two progenitor channels that are known to produce GRBs: mergers of compact objects (merger GRBs) or the collapse of massive stars (collapsar GRBs). In particular, the merger GRB population may also include bursts with a short, hard < 2 s spike and subsequent longer, softer extended emission. The recent discovery of a kilonova-the radioactive glow of heavy elements made in neutron star mergers-in the 50-s-duration GRB 211211A further demonstrates that mergers can drive long, complex GRBs that mimic the collapsar population. Here we present a detailed temporal and spectral analysis of the high-energy emission of GRB 211211A. We demonstrate that the emission has a purely synchrotron origin, with both the peak and cooling frequencies moving through the gamma-ray band down to X-rays, and that the rapidly evolving spectrum drives the extended emission signature at late times. The identification of such spectral evolution in a merger GRB opens avenues to diagnostics of the progenitor type.

Automated summary

The classification of long and short gamma-ray bursts (GRBs) does not perfectly correspond to the two known progenitor channels: merger GRBs and collapsar GRBs. The recent discovery of a kilonova in a long-duration GRB 211211A further confirms that mergers can produce long, complex GRBs similar to collapsar population. A detailed temporal and spectral analysis of the high-energy emission of GRB 211211A reveals a purely synchrotron origin and shows that the rapidly evolving spectrum is responsible for the extended emission signature at late times, providing new diagnostics for the progenitor type in merger GRBs.