Radio Constraints on $r$-process Nucleosynthesis by Collapsars

TL;DR

This study uses radio observations of gamma-ray bursts to constrain the role of collapsars in producing heavy elements via the r-process, finding limited contribution compared to other sources like neutron star mergers.

Contribution

First observational constraints on collapsar r-process nucleosynthesis using radio follow-up of nearby long gamma-ray bursts.

Findings

No excess radio emission detected, limiting collapsar r-process contribution to less than 0.2 solar masses.

Results challenge the idea that collapsars are the dominant r-process sites, but uncertainties remain due to modeling limitations.

Constraints are more stringent under models with constant circum-merger densities.

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 multi-messenger 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. 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/BAT GRBs located within 2 Gpc. No radio counterpart was identified in excess of the radio afterglow of the GRBs in our sample, limiting the collapsar $r$-process contribution to $\lesssim0.2$ M$_\odot$ under the models we considered, with constant circum-merger densities giving more stringent constraints. While our results are in tension with collapsars being the majority $r$-process production sites, the ejecta mass and velocity profile of collapsar winds is not yet well modeled. As such, our results are currently subject to large uncertainties, but further theoretical work could greatly improve them.