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 well sampled light curves to infer the burst isotropic equivalent energies and circumburst densities. For this subset, we find median isotropic equivalent gamma ray and kinetic energies of E_gamma,iso~2x10^51 erg, and E_K,iso~(1 3)x10^51 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)x10^ 3 cm^ 3, and that ~80 95% of bursts have densities of less than 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 theta_jet=16+ 10 deg. We also calculate a median beaming factor of f_b~0.04, leading to a beaming corrected total energy release of E_true~1.6x10^50 erg. Furthermore, we calculate a beaming corrected event rate of R_true=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.