Low frequency view of GW 170817/GRB 170817A with the Giant Meterwave Radio Telescope

TL;DR

This study presents low-frequency radio observations of GW 170817/GRB 170817A, revealing afterglow emergence around 60 days post-burst and constraining jet structure and viewing angle through modeling and MCMC analysis.

Contribution

First low-frequency radio observations of GW 170817/GRB 170817A, combined with multi-wavelength data, to model jet structure and estimate viewing angles using MCMC methods.

Findings

Afterglow emerged at ~60 days at low frequencies.

Radio light curve evolution is consistent with higher frequency trends.

Bounds on viewing angle align with gravitational wave estimates.

Abstract

The short gamma-ray burst (GRB) 170817A was the first GRB associated with a gravitational-wave event. Due to the exceptionally low luminosity of the prompt $\gamma$-ray and the afterglow emission, the origin of both radiation components is highly debated. The most discussed models for the burst and the afterglow include a regular GRB jet seen off-axis and the emission from the cocoon encompassing a "choked" jet. Here, we report low radio-frequency observations at 610 and 1390~MHz obtained with the Giant Metrewave Radio Telescope (GMRT). Our observations span a range of $\sim7$ to $\sim152$ days after the burst. The afterglow started to emerge at these low frequencies about 60~days after the burst. The $1390$~MHz light curve barely evolved between 60 and 150 days, but its evolution is also marginally consistent with a $F_\nu\propto t^{0.8}$ rise seen in higher frequencies. We model the radio data and archival X-ray, optical and high-frequency radio data with models of top-hat and Gaussian structured GRB jets. We performed a Markov Chain Monte Carlo analysis of the structured-jet parameter space. Though highly degenerate, useful bounds on the posterior probability distributions can be obtained. Our bounds of the viewing angle are consistent with that inferred from the gravitational wave signal. We estimate the energy budget in prompt emission to be an order of magnitude lower than that in the afterglow blast-wave.