2018ApJ...858...65L

We present the first results from a recently concluded study of GRBs at z >= 5 with the Karl G. Jansky Very Large Array (VLA). Spanning 1 to 85.5 GHz and 7 epochs from 1.5 to 82.3 days, our observations of GRB 140311A are the most detailed joint radio and millimeter observations of a GRB afterglow at z >= 5 to date. In conjunction with optical/near-IR and X-ray data, the observations can be understood in the framework of radiation from a single blast wave shock with energy E_K,iso~8.5×10⁵³ erg expanding into a constant density environment with density, n₀~8cm^–3. The X-ray and radio observations require a jet break at t_jet~0.6 days, yielding an opening angle of θ_jet~4^○ and a beaming-corrected blast wave kinetic energy of E_K~2.2×10⁵⁰ erg. The results from our radio follow-up and multiwavelength modeling lend credence to the hypothesis that detected high-redshift GRBs may be more tightly beamed than events at lower redshift. We do not find compelling evidence for reverse shock emission, which may be related to fast cooling driven by the moderately high circumburst density.