Low frequency view of GRB 190114C reveals time varying shock micro-physics

TL;DR

This study presents long-term radio and optical observations of GRB 190114C, revealing complex afterglow behavior and evolving shock microphysics, challenging standard models and highlighting the importance of ambient medium assumptions.

Contribution

It provides the first detailed low-frequency view of GRB 190114C's afterglow, demonstrating time-varying shock microphysics and the impact of ambient medium profiles on energy estimates.

Findings

Afterglow does not follow standard spectral and temporal relations.

Microphysical parameters evolve with time.

Energy estimates depend on ambient medium assumptions.

Abstract

We present radio and optical afterglow observations of the TeV-bright long Gamma Ray Burst (GRB) 190114C at a redshift of $z=0.425$, which was detected by the MAGIC telescope. Our observations with ALMA, ATCA, and uGMRT were obtained by our low frequency observing campaign and range from $\sim1$ to $\sim140$ days after the burst and the optical observations were done with three optical telescopes spanning up to $\sim25$ days after the burst. Long term radio/mm observations reveal the complex nature of the afterglow, which does not follow the spectral and temporal closure relations expected from the standard afterglow model. We find that the microphysical parameters of the external forward shock, representing the share of shock-created energy in the non-thermal electron population and magnetic field, are evolving with time. The inferred kinetic energy in the blast-wave depends strongly on the assumed ambient medium density profile, with a constant density medium demanding almost an order of magnitude higher energy than in the prompt emission, while a stellar wind-driven medium requires approximately the same amount energy as in prompt emission.