A DECADE OF SHORT-DURATION GAMMA-RAY BURST BROADBAND AFTERGLOWS: ENERGETICS, CIRCUMBURST DENSITIES, AND JET OPENING ANGLES

We present a comprehensive catalog and analysis of broad-band afterglow observations for 103 short duration gamma-ray bursts (GRBs), comprised of all short GRBs from November 2004 to March 2015 with prompt follow-up observations in the X-ray, optical, near-infrared and/or radio bands. These afterglow observations have uncovered 71 X-ray detections, 30 optical/NIR detections, and 4 radio detections. Employing the standard afterglow synchrotron model, we perform joint probability analyses for a subset of 38 short GRBs with wellsampled light curves to infer the burst isotropic-equivalent energies and circumburst densities. For this subset, we find median isotropic-equivalent γ-ray and kinetic energies of Eγ,iso ≈ 2 × 1051 erg, and EK,iso ≈ (1 − 3) × 1051 erg, respectively, depending on the values of the model input parameters. We further find that short GRBs occur in low-density environments, with a median density of n ≈ (3 − 15) × 10−3 cm−3 , and that ≈ 80 − 95% of bursts have densities of n . 1 cm−3. We investigate trends between the circumburst densities and host galaxy properties, and find that events located at large projected offsets of & 10 effective radii from their hosts exhibit particularly low densities of n . 10−4 cm−3 , consistent with an IGM-like environment. Using late-time afterglow data for 11 events, we find a median jet opening angle of θj = 16±10◦ . We also calculate a median beaming factor of fb ≈ 0.04, leading to a beaming-corrected total energy release of Etrue ≈ 1.6 × 1050 erg. Furthermore, we calculate a beaming-corrected event rate of ℜtrue = 270+1580 −180 Gpc−3 yr−1 , or ≈ 8 +47 −5 yr−1 within a 200 Mpc volume, the Advanced LIGO/Virgo typical detection distance for NS-NS binaries.