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 ∼0.5 day 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 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 ∼(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 E γ ∼ (0.5-2) × 10⁴⁹ erg and E _K ∼ (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 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.