TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3798 SUBJECT: GRB 050813: Magellan detection of a high redshift galaxy cluster DATE: 05/08/14 05:28:50 GMT FROM: Edo Berger at Carnegie Obs Mike Gladders, Edo Berger (Carnegie Observatories), Nidia Morrell, and Miguel Roth (Las Campanas Observatory) report: "We imaged the field of GRB 050813 (GCN 3788) 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) 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)." Further observations are planned.