Short Grb 130603b: Discovery of a Jet Break in the Optical and Radio Afterglows, and a Mysterious Late-Time X-Ray Excess

Abstract

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.