VLBI and Archival VLA and WSRT Observations of the GRB 030329 Radio Afterglow

TL;DR

This study combines VLBI and archival radio observations to analyze the long-term afterglow of GRB 030329, revealing a simple decay pattern and supporting an ISM-like environment with no evidence of counter jet rebrightening.

Contribution

The paper introduces a semi-analytic modeling method for GRB afterglows and applies it to derive burst properties and circumburst medium characteristics from extensive radio data.

Findings

Radio afterglow decays as a simple power law at 5 GHz.

Unresolved source size at day 2032 supports an ISM-like medium.

Proper motion limit supports the standard afterglow model.

Abstract

We present VLBI and archival Karl G. Jansky Very Large Array (VLA) and Westerbork Synthesis Radio Telescope (WSRT) observations of the radio afterglow from the gamma-ray burst (GRB) of 2003 March 29 (GRB 030329) taken between 672 and 2032 days after the burst. The EVLA and WSRT data suggest a simple power law decay in the flux at 5 GHz, with no clear signature of any rebrightening from the counter jet. We report an unresolved source at day 2032 of size $1.18\pm0.13$ mas, which we use in conjunction with the expansion rate of the burst to argue for the presence of a uniform, ISM-like circumburst medium. We develop a semi-analytic method to model gamma-ray burst afterglows, and apply it to the 5 GHz light curve to perform burst calorimetry. A limit of $< 0.067$ mas yr$^{-1}$ is placed on the proper motion, supporting the standard afterglow model for gamma-ray bursts.