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 GRB 130603B, and uncover a break in the radio and optical bands at ≈ 0.5 d after the burst, best explained as a jet break with an inferred jet opening angle of ≈ 4 − 8. GRB 130603B is only the third short GRB with a radio afterglow detection to date, and the first time that a jet break is 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 ≈ (0.6 − 1.7)×1051 erg and a circumburst density of ≈ 5×10−3 − 30 cm−3. From the inferred opening angle of GRB 130603B, we calculate beaming-corrected energies of Eγ ≈ (0.5 − 2)× 1049 erg and EK ≈ (0.1 − 1.6)× 1049 erg. Along with previous measurements and lower limits we find a median opening angle of ≈ 10. Using the all-sky observed rate of 10 Gpc−3 yr−1, this implies a true short GRB rate of ≈ 20 yr−1 within 200 Mpc, the Advanced LIGO/VIRGO sensitivity range for neutron star binary mergers. Finally, we uncover evidence for significant excess emission in the X-ray afterglow of GRB 130603B at & 1 d 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.