The Afterglow, Energetics, and Host Galaxy of the Short‐Hard Gamma‐Ray Burst 051221a

We present detailed optical, X-ray and radio observations of the brightafterglow 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 theredshift of the burst, z=0.5464. At this redshift the isotropic-equivalentprompt energy release was about 1.5 x 10^51 erg, and using the standardafterglow synchrotron model we find that the blastwave kinetic energy issimilar, E_K,iso ~ 8.4 x 10^51 erg. An observed jet break at t ~ 5 daysindicates that the opening angle is ~ 7 degrees and the total beaming-correctedenergy is therefore ~ 2.5 x 10^49 erg, comparable to the values inferred forprevious short GRBs. We further show that the burst experienced an episode ofenergy injection by a factor of 3.4 between t=1.4 and 3.4 hours, which wasaccompanied by reverse shock emission in the radio band. This result providescontinued evidence that the central engines of short GRBs may be activesignificantly longer than the duration of the burst and/or produce a wide rangeof Lorentz factors. Finally, we show that the host galaxy of GRB051221a isactively forming stars at a rate of about 1.6 M_solar/yr, but at the same timeexhibits evidence for an appreciable population of old stars (~ 1 Gyr) and nearsolar metallicity. The lack of bright supernova emission and the lowcircumburst density (n ~ 10^-3 cm^-3) continue to support the idea that shortbursts are not related to the death of massive stars and are instead consistentwith a compact object merger. Given that the total energy release is a factorof ~ 10 larger than the predicted yield for a neutrino annihilation mechanism,this suggests that magnetohydrodynamic processes may be required to power theburst.Comment: Final version (to appear in ApJ on 20 September 2006