2005 December 21.0773 UT, the space-based Swift Gamma Ray Burst Explorer (Burst Alert Telescope) detected a classical short gamma ray burst. The total duration of this event (named GRB 051221a) was about 1.4 seconds. Alicia M. Soderberg (CALTECH) and her collaborators have studied the burst and its afterglow in great detail using several datasets provided by space observations, radio observations with the VLA and optical observations with Gemini. They derived a detailed set of physical parameters for this burst and their analysis helps to better understand the nature of the progenitor of short bursts and their host environments.

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Credit Gemini Observatory/AURA

The current observations by the Soderberg team supports the trend that progenitors of short burst GRBs arise from old stellar populations.

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Title: The Afterglow, Energetics and Host Galaxy of the Short-Hard Gamma-Ray Burst 051221a
Authors: A. M. Soderberg, E. Berger, M. Kasliwal, D. A. Frail, P. A. Price, B. P. Schmidt, S. R. Kulkarni, D. B. Fox, S. B. Cenko, A. Gal-Yam, E. Nakar, K. C. Roth

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 10⁵¹ erg, and using the standard afterglow synchrotron model we find that the blastwave kinetic energy is similar, E_K,iso ~ 8.4 x 10⁵¹ 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 10⁴⁹ 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.

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