A VLA study of high-redshift GRBs. II. GRB140304A

The radio and millimeter data exhibit unusual multiple spectral components, which cannot be simply explained by standard forward and reverse shock scenarios. Through detailed multiwavelength analysis spanning radio to X-rays, we constrain the forward shock parameters to E_k,iso_~4.9x10^54^erg, A_*_~2.6x10^-2^, {epsilon}_e_~2.5x10^-2^, {epsilon}_B_~5.9x10^-2^, p~2.6, and {theta}_jet_~1.1{deg}, yielding a beaming-corrected {gamma}-ray and kinetic energy, E_{gamma}_~2.3x10^49^erg and E_K_~9.5x10^50^erg, respectively.

We model the excess radio emission as due to a combination of a late-time reverse shock (RS) launched by a shell collision, which also produces a rebrightening in the X-rays at ~0.26days, and either a standard RS or diffractive interstellar scintillation (ISS). Under the standard RS interpretation, we invoke consistency arguments between the forward and reverse shocks to derive a deceleration time, t_dec_~100s, the ejecta Lorentz factor, {Gamma}(t_dec_)~300, and a low RS magnetization, R_B_~0.6.