Short GRB 130603B: Discovery of a jet break in the optical and radio afterglows, and a mysterious late-time X-ray excess

摘要

We present radio, optical/NIR, and X-ray observations of the afterglow of theshort-duration 130603B, and uncover a break in the radio and optical bands at0.5 d after the burst, best explained as a jet break with an inferred jetopening angle of 4-8 deg. GRB 130603B is only the third short GRB with a radioafterglow detection to date, and the first time that a jet break is evident inthe radio band. We model the temporal evolution of the spectral energydistribution to determine the burst explosion properties and find anisotropic-equivalent kinetic energy of (0.6-1.7) x 10^51 erg and a circumburstdensity of 5 x 10^-3-30 cm^-3. From the inferred opening angle of GRB 130603B,we calculate beaming-corrected energies of Egamma (0.5-2) x 10^49 erg and EK(0.1-1.6) x 10^49 erg. Along with previous measurements and lower limits wefind a median short GRB opening angle of 10 deg. Using the all-sky observedrate of 10 Gpc^-3 yr^-1, this implies a true short GRB rate of 20 yr^-1 within200 Mpc, the Advanced LIGO/VIRGO sensitivity range for neutron star binarymergers. Finally, we uncover evidence for significant excess emission in theX-ray afterglow of GRB 130603B at >1 d and conclude that the additional energycomponent could be due to fall-back accretion or spin-down energy from amagnetar formed following the merger.