TITLE:   GCN CIRCULAR
NUMBER:  8914
SUBJECT: GRB 081211B: Nearby cluster
DATE:    09/02/23 05:25:22 GMT
FROM:    Daniel Perley at U.C. Berkeley  <dperley@astro.berkeley.edu>

D. A. Perley, J. S. Bloom, and N. R. Butler (UC Berkeley) report:

GRB 081211B was discovered in the Swift-BAT slew survey (Copete et al, 
GCN 8661) and, based on observations by Konus-Wind (Golenetskii et al., 
GCN 8676), was classified as a possible short burst with an extended 
emission component.  We note that the GRB localization lies within a 
visual galaxy overdensity in SDSS archival imaging, and near the centers 
of several reported clusters in the literature, which likely correspond 
to the same physically extended structure: ZW 3893, Abell 1196, and 
MaxBCG J168.22310+53.83028.

Redshifts from the Sloan Digital Sky Survey of the brightest two 
apparent cluster members place this cluster at a probable redshift of 
z=0.216.

On the night of 2009-02-19 (UT) we observed the field with Keck I (+ 
LRIS) for an exposure time of 990 sec (g-band) and 870 sec (R-band) 
simultaneously through thin cloud cover.  No host galaxy underlying the 
XRT position (Page et al., GCN 8666) is detected to approximately R > 
25, g > 26 mag.  The nearest catalogued objects are:

SDSS J111303.09+534953.8  g=21.26 r=20.26 i=20.03 (7" = 24 kpc)
SDSS J111304.73+534959.5  g=20.56 r=19.25 i=18.84 (16" = 56 kpc)

An additional very faint extended object is located 3" west of the 
center of the XRT position, outside the 90% confidence error circle. 
Images of the field (from our observations and from SDSS) are posted to:

http://lyra.berkeley.edu/~dperley/081211b/081211b_keck.png 
http://lyra.berkeley.edu/~dperley/081211b/081211b_sdss.png

If the BAT detection represents extended emission from this event and no 
fainter host galaxy is found, this would constitute an additional case 
of a short GRB event with detected extended emission occurring without a 
coincident host galaxy (after GRB 080503 - arXiv:0811.1044), as well as 
an additional example of a short burst occurring within a galaxy cluster 
(see also e.g. GRBs 050509B, 050813, 051210, 061201).  The isotropic 
gamma-ray energy release at the cluster redshift would be 7 x 10^49 erg 
in the BAT bandpass, comparable to values measured for other short GRBs.

We encourage deeper observations of the field.