GCN Circular 3798

Mike Gladders, Edo Berger (Carnegie Observatories), Nidia Morrell, and
Miguel Roth (Las Campanas Observatory) report:

"We imaged the field of GRB 050813 (GCN 3788
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 ) with the PANIC IR camera on
the Magellan/Baade 6.5-m telescope starting on 2005 Aug. 14.017 UT.
Observations were obtained with the K-s and Y-Carnegie filters, under poor
conditions (variable thick clouds, bright moonlight, and poor seeing of
~1").  Initial processing of these data suggest that this line of sight is
centered on a high-redshift galaxy cluster.  A composite Y-K color image,
as well as a greyscale image of the stacked K frame is shown at:

        http://www.ociw.edu:/~eberger/grb050813_panic.jpg

The apparent centrally concentrated population of faint galaxies with red
and uniform colors is strongly suggestive of a galaxy cluster, and looks
similar to extensive data taken with this instrument on known high-z
galaxy clusters from the Red-Sequence Cluster Survey (Gladders & Yee
2005). The apparent size of the core of this distribution (~1 arcmin) and
the magnitude of the objects suggest this putative cluster is at high
redshift (above z=0.5, and likely at z~1). The objects noted as B and C by
Gorosabel et al. (GCN 3796
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 ) in the XRT error circle are both red and
likely cluster ellipticals, and both are approximately K=19, based on an
initial calibration of the image to the 2MASS magnitude of the bright star
to the immediate SE of these objects.

Given the low probability of a chance coincidence we suggest that GRB
050813 most likely occurred in the cluster environment.  This supports
recent associations of short GRBs with old galaxies (050509b: Bloom et al.
astro-ph/0505480; 050724: Berger et al. astro-ph/0508115).  The likely
higher redshift compared to the latter two events may also explain the
faintness of the X-ray afterglow (GCN 3790
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 )."

Further observations are planned.