The signature of the central engine in the weakest relativistic explosions: GRB100316D

TL;DR

This study analyzes late-time radio and X-ray data of GRB100316D, revealing it as a weak, central-engine driven explosion with properties intermediate between typical GRBs and supernovae, possibly involving a rapidly rotating magnetar or black hole accretion.

Contribution

It provides detailed constraints on the explosion energy, ejecta velocity, and medium environment, highlighting the presence of a central engine in a weak GRB, which is a novel insight.

Findings

Ejecta energy ~10^49 erg with mildly relativistic speeds

Detection of excess soft X-ray emission at late times

Identification of a central engine in a weak GRB

Abstract

We present late-time radio and X-ray observations of the nearby sub-energetic Gamma-Ray Burst (GRB)100316D associated with supernova (SN) 2010bh. Our broad-band analysis constrains the explosion properties of GRB100316D to be intermediate between highly relativistic, collimated GRBs and the spherical, ordinary hydrogen-stripped SNe. We find that ~10^49 erg is coupled to mildly-relativistic (Gamma=1.5-2), quasi-spherical ejecta, expanding into a medium previously shaped by the progenitor mass-loss with rate Mdot ~10^-5 Msun yr^-1 (for wind velocity v_w = 1000 km s^-1). The kinetic energy profile of the ejecta argues for the presence of a central engine and identifies GRB100316D as one of the weakest central-engine driven explosions detected to date. Emission from the central engine is responsible for an excess of soft X-ray radiation which dominates over the standard afterglow at late times (t>10 days). We connect this phenomenology with the birth of the most rapidly rotating magnetars. Alternatively, accretion onto a newly formed black hole might explain the excess of radiation. However, significant departure from the standard fall-back scenario is required.