1000 days of lowest frequency emission from the low-luminosity GRB 171205A

TL;DR

This study presents the first low-frequency radio observations of GRB 171205A over 1000 days, revealing a stratified wind-like environment and evidence for a dual-component afterglow involving a jet and cocoon.

Contribution

It provides the first detection of a GRB afterglow at 250--500 MHz and analyzes the environment and emission components using extensive low-frequency data.

Findings

Detected GRB afterglow at 250--500 MHz for the first time.

Found the circumburst medium to be stratified, not constant density.

Identified two emission components: a weak jet and a surrounding cocoon.

Abstract

We report the lowest frequency measurements of gamma-ray burst (GRB) 171205A with the upgraded Giant Metrewave Radio Telescope (uGMRT) covering a frequency range from 250--1450 MHz and a period of $4-937$ days. It is the first GRB afterglow detected at 250--500 MHz frequency range and the second brightest GRB detected with the uGMRT. Even though the GRB is observed for nearly 1000 days, there is no evidence of transition to non-relativistic regime. We also analyse the archival ${\it Chandra}$ X-ray data on day $\sim 70$ and day $\sim 200$. We also find no evidence of a jet break from the analysis of combined data. We fit synchrotron afterglow emission arising from a relativistic, isotropic, self-similar deceleration as well as from a shock-breakout of wide-angle cocoon. Our data also allow us to discern the nature and the density of the circumburst medium. We find that the density profile deviates from a standard constant density medium and suggests that the GRB exploded in a stratified wind like medium. Our analysis shows that the lowest frequency measurements covering the absorbed part of the light curves are critical to unravel the GRB environment. Our data combined with other published measurements indicate that the radio afterglow has contribution from two components: a weak, possibly slightly off-axis jet and a surrounding wider cocoon, consistent with the results of Izzo et al. (2019). The cocoon emission likely dominates at early epochs, whereas the jet starts to dominate at later epochs, resulting in flatter radio lightcurves.