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Understanding The Nature of a Distant Short GRB

September 29, 2006

Fig. 1: Color composite image of the field of GRB 051221a as observed with Gemini GMOS-North. The host galaxy of the short gamma ray burst is the blue source in the white circle (Swift/XRT location box). The blue color of the object indicates a star forming galaxy.

Fig. 2: Residual image of the optical afterglow (r'-band from GMOS-North) of GRB 051221a highlighted by image subtraction techniques. The position of the afterglow (cross, template image) is offset by 0.12 +/- 0.04 arcseconds with respect to the host galaxy's center.

Fig. 3: . GMOS spectrum of the host galaxy of the short gamma ray burst GRB 051221a. Several bright emission lines are detected indicating a redshift of z = 0.5464.

On 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.

The Gemini North Telescope staff was alerted and began observations about 2.8 hours after the burst using the Gemini Multi-Object Spectrograph (GMOS). It detected a bright optical source (Figure 1), and the evolution of this afterglow was followed over several days (Figure 2). The optical source detected by GMOS corresponds to the afterglow that faded relatively slowly. This afterglow is produced by the dynamical interaction of the ejecta with the surrounding medium.

An optical spectrum of the host galaxy was also obtained (Figure 3), which allowed for the redshift (distance) to be derived and it was found to be z = 0.5464. This makes GRB 051221a the most distant short gamma ray burst observed so far with a robust redshift determination. The emission lines indicate that the host galaxy is actively forming stars at a rate of about 1.6 Msun per year. This is higher than what has been found in all previous short gamma ray burst hosts. Well-known absorption features like the Ca II H and K lines are also apparent and these are indicative of an appreciably old stellar population. The mean metallicity of the host galaxy is nearly the same as our sun.

Gamma ray bursts are among the most energetic explosions in the whole universe. Gamma ray burst have been known since 1967 when they were first detected serendipitously by the Vela spacecrafts built to detect man-made nuclear explosions in the upper atmosphere or in near-Earth space. It was only 30 years later, in 1997, that their origins have been associated with very energetic events happening in distant galaxies. There are two classes of gamma ray burst: the long soft burst (surmised to be associated with the explosion of massive stars into supernovae) and the short hard bursts, (like GRB 051221a), thought to be the catastrophic energy output of merging massive stellar remnants, like neutron stars or black holes.

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

For more details, see the paper "The Afterglow, Energetics and Host Galaxy of the Short-Hard Gamma-Ray Burst 051221a", by Alicia M. Soderberg et al., 2006, The Astrophysical Journal, in press (October 10 issue).