SHORT GRB 130603B: DISCOVERY OF A JET BREAK IN THE OPTICAL AND RADIO AFTERGLOWS, AND A MYSTERIOUS LATE-TIME X-RAY EXCESS

We present radio, optical/NIR, and X-ray observations of the afterglow of the short-duration Swift and Konus-Wind GRB 130603B, and uncover a break in the radio and optical bands at approximate to 0.5 day after the burst, best explained as a jet break with an inferred jet opening angle of approximate to 4 degrees-8 degrees. GRB 130603B is only the third short GRB with a radio afterglow detection to date, and represents the first time that a jet break has been evident in the radio band. We model the temporal evolution of the spectral energy distribution to determine the burst explosion properties and find an isotropic-equivalent kinetic energy of approximate to (0.6-1.7) x 10(51) erg and a circumburst density of approximate to 5x10(-3)-30 cm(-3). From the inferred opening angle of GRB 130603B, we calculate beaming-corrected energies of E gamma approximate to (0.5-2) x 10(49) erg and E-K approximate to (0.1-1.6) x 10(49) erg. Along with previous measurements and lower limits we find a median opening angle of approximate to 10 degrees. Using the all-sky observed rate of 10 Gpc(-3) yr(-1), this implies a true short GRB rate of approximate to 20 yr(-1) within 200 Mpc, the Advanced LIGO/VIRGO sensitivity range for neutron star binarymergers. Finally, we uncover evidence for significant excess emission in the X-ray afterglow of GRB 130603B at greater than or similar to 1 day and conclude that the additional energy component could be due to fall-back accretion or spin-down energy from a magnetar formed following the merger.