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
The core collapse of rapidly rotating massive ∼ 10
- NSF-PAR ID:
- 10385620
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 941
- Issue:
- 1
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- Article No. 100
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract r -process) following the merger. Recent simulations suggest that the “collapsar” SNe that trigger long GRBs may also producer -process elements. While observations of GRB 211211A and its afterglow rule out an SN typical of those that follow long GRBs, an unusual collapsar could explain both the duration of GRB 211211A and ther -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 of56Ni 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. -
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Gamma-ray bursts (GRBs) are among the brightest and most energetic events in the universe. The duration and hardness distribution of GRBs has two clusters, now understood to reflect (at least) two different progenitors. Short-hard GRBs (SGRBs; T90 <2 s) arise from compact binary mergers, while long-soft GRBs (LGRBs; T90 >2 s) have been attributed to the collapse of peculiar massive stars (collapsars). The discovery of SN 1998bw/GRB 980425 marked the first association of a LGRB with a collapsar and AT 2017gfo/GRB 170817A/GW170817 marked the first association of a SGRB with a binary neutron star merger, producing also gravitational wave (GW). Here, we present the discovery of ZTF20abwysqy (AT2020scz), a fast-fading optical transient in the Fermi Satellite and the InterPlanetary Network (IPN) localization regions of GRB 200826A; X-ray and radio emission further confirm that this is the afterglow. Follow-up imaging (at rest-frame 16.5 days) reveals excess emission above the afterglow that cannot be explained as an underlying kilonova (KN), but is consistent with being the supernova (SN). Despite the GRB duration being short (rest-frame T90 of 0.65 s), our panchromatic follow-up data confirms a collapsar origin. GRB 200826A is the shortest LGRB found with an associated collapsar; it appears to sit on the brink between a successful and a failed collapsar. Our discovery is consistent with the hypothesis that most collapsars fail to produce ultra-relativistic jets.more » « less
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Abstract The heaviest elements in the universe are synthesized through rapid neutron capture ( r -process) in extremely neutron-rich outflows. Neutron star mergers were established as an important r -process source through the multimessenger observation of GW170817. Collapsars were also proposed as a potentially major source of heavy elements; however, this is difficult to probe through optical observations due to contamination by other emission mechanisms. Here we present observational constraints on r -process nucleosynthesis by collapsars based on radio follow-up observations of nearby long gamma-ray bursts (GRBs). We make the hypothesis that late-time radio emission arises from the collapsar wind ejecta responsible for forging r -process elements, and consider the constraints that can be set on this scenario using radio observations of a sample of Swift/Burst Alert Telescope GRBs located within 2 Gpc. No radio counterpart was identified in excess of the radio afterglow of the GRBs in our sample. This gives the strictest limit to the collapsar r -process contribution of ≲0.2 M ⊙ for GRB 060505 and GRB 05826, under the models we considered. Our results additionally constrain energy injection by a long-lived neutron star remnant in some of the considered GRBs. While our results are in tension with collapsars being the majority of r -process production sites, the ejecta mass and velocity profile of collapsar winds, and the emission parameters, are not yet well modeled. As such, our results are currently subject to large uncertainties, but further theoretical work could greatly improve them.more » « less
