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.Comment: Final version (to appear in ApJ on 20 September 2006
