A VLA Study of High-redshift GRBs. II. The Complex Radio Afterglow of GRB 140304A: Shell Collisions and Two Reverse Shocks

Abstract

We present detailed multifrequency, multiepoch radio observations of GRB. 140304A at z = 5.283 from 1 to 86 GHz and from 0.45 to 89 days. 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,E- iso approximate to 4.9 x 10(54) erg, A(*) approximate to 2.6 x 10(-2), epsilon(e) approximate to 2.5 x 10(-2), epsilon(B) approximate to 5.9 x 10(-2), p approximate to 2.6, and theta(jet) approximate to 1 degrees.1, yielding a beaming-corrected gamma-ray and kinetic energy, E gamma approximate to 2.3 x 10(49) erg and E-K approximate to 9.5 x 10(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 approximate to 0.26 days, 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) approximate to 100 s, the ejecta Lorentz factor, Gamma(t(dec)) approximate to 300, and a low RS magnetization, R-B approximate to 0.6. Our observations highlight both the power of radio observations in capturing RS emission and thus constraining the properties of GRB ejecta and central engines and the challenge presented by ISS in conclusively identifying RS emission in GRB radio afterglows.