A collapsar origin for GRB 211211A is (just barely) possible

TL;DR

This paper explores whether a low-mass collapsar could be the progenitor of GRB 211211A, challenging the typical binary neutron-star merger origin, by modeling its afterglow and ejecta properties.

Contribution

It introduces semianalytic radiation transport models for low-mass collapsars as potential progenitors of long GRBs like GRB 211211A, providing alternative explanations for observed features.

Findings

Best-fit models have high kinetic energies and unusual Ni-56 enrichment patterns.

Radio observations can distinguish collapsar ejecta from kilonova signatures.

Collapsar models can potentially explain the long duration and r-process features of GRB 211211A.

Abstract

Gamma-ray bursts (GRBs) have historically been divided into two classes. Short-duration GRBs are associated with binary neutron-star mergers (NSMs), while long-duration bursts are connected to a subset of core-collapse supernovae (SNe). GRB 211211A recently made headlines as the first long-duration burst purportedly generated by an NSM. The evidence for an NSM origin was excess optical and near-infrared emission consistent with the kilonova observed after the gravitational wave-detected NSM GW170817. Kilonovae derive their unique electromagnetic signatures from the properties of the heavy elements synthesized by rapid neutron capture (the r-process) following the merger. Recent simulations suggest that the "collapsar" SNe that trigger long GRBs may also produce r-process elements. While observations of GRB 211211A and its afterglow ruled out an SN typical of those that follow long GRBs, an unusual collapsar could explain both the duration of GRB 211211A and the r-process-powered excess in its afterglow. We use semianalytic radiation transport modeling to evaluate low-mass collapsars as the progenitors of GRB 211211A-like events. We compare a suite of collapsar models to the afterglow-subtracted emission that followed GRB 211211A, and find the best agreement for models with high kinetic energies and an unexpected pattern of Ni-56 enrichment. We discuss how core-collapse explosions could produce such ejecta, and how distinct our predictions are from those generated by more straightforward kilonova models. We also show that radio observations can distinguish between kilonovae and the more massive collapsar ejecta we consider here.