Early-time VLA observations and broad-band afterglow analysis of the Fermi-LAT detected GRB 130907A

TL;DR

This study presents early radio and broad-band afterglow observations of GRB 130907A, analyzing its environment and shock components, highlighting the importance of early radio data for understanding GRB physics.

Contribution

It provides one of the earliest radio follow-ups of a GRB with VLA and models the afterglow with a wind-to-ISM transition, offering insights into the burst's environment and shock signatures.

Findings

No unambiguous reverse shock signature detected.

Broad-band data consistent with a forward-shock model with wind-ISM transition.

Derived physical parameters are within typical ranges, except for a higher wind density parameter.

Abstract

We present multi-wavelength observations of the hyper-energetic gamma-ray burst (GRB) 130907A, a Swift-discovered burst with early radio observations starting at $\approx 4$ hr after the $\gamma$-ray trigger. GRB 130907A was also detected by the Fermi/LAT instrument and, at late times, showed a strong spectral evolution in X-rays. We focus on the early-time radio observations, especially at $>10 $ GHz, to attempt identifying reverse shock signatures. While our radio follow-up of GRB 130907A ranks among the earliest observations of a GRB with the Karl G. Jansky Very Large Array (VLA), we did not see an unambiguous signature of a reverse shock. While a model with both reverse and forward-shock can correctly describe the observations, the data is not constraining enough to decide upon the presence of the reverse-shock component. We model the broad-band data using a simple forward-shock synchrotron scenario with a transition from a wind environment to a constant density interstellar medium (ISM) in order to account for the observed features. Within the confines of this model, we also derive the underlying physical parameters of the fireball, which are within typical ranges except for the wind density parameter ($A_*$), which is higher than those for bursts with wind-ISM transition, but typical for the general population of bursts. We note the importance of early-time radio observations of the afterglow (and of well sampled light curves) to unambiguously identify the potential contribution of the reverse shock.