The Afterglow and Host Galaxy of the Energetic Short-Hard Gamma-Ray Burst 051221

We present detailed optical, X-ray and radio observations of the bright afterglow of the short gamma-ray burst 051221a obtained with Gemini, Swift/XRT, and the Very Large Array, as well as optical spectra from which we measure the redshift of the burst, z=0.5464. At this redshift the isotropic-equivalent prompt energy release was about 1.5 x 10^51 erg, and using the standard afterglow synchrotron model we find that the blastwave kinetic energy is similar, E_K,iso ~ 8.4 x 10^51 erg. An observed jet break at t ~ 5 days indicates that the opening angle is ~ 7 degrees and the total beaming-corrected energy is therefore ~ 2.5 x 10^49 erg, comparable to the values inferred for previous short GRBs. We further show that the burst experienced an episode of energy injection by a factor of 3.4 between t=1.4 and 3.4 hours, which was accompanied by reverse shock emission in the radio band. This result provides continued evidence that the central engines of short GRBs may be active significantly longer than the duration of the burst and/or produce a wide range of Lorentz factors. Finally, we show that the host galaxy of GRB051221a is actively forming stars at a rate of about 1.6 M_solar/yr, but at the same time exhibits evidence for an appreciable population of old stars (~ 1 Gyr) and near solar metallicity. The lack of bright supernova emission and the low circumburst density (n ~ 10^-3 cm^-3) continue to support the idea that short bursts are not related to the death of massive stars and are instead consistent with a compact object merger. Given that the total energy release is a factor of ~ 10 larger than the predicted yield for a neutrino annihilation mechanism, this suggests that magnetohydrodynamic processes may be required to power the burst.