TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  5387
SUBJECT: GRB 051109B:  Bright spiral host galaxy at low redshift
DATE:    06/08/02 04:51:03 GMT
FROM:    Daniel Perley at U.C. Berkeley  <dperley@astro.berkeley.edu>

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

On the night of 2006 July 25 (UT), we imaged the field of GRB051109B 
(GCN 4222) for 630 seconds in g' and 600 seconds in R using Keck I (+ 
LRIS).  There is a bright (R ~ 14.5) spiral galaxy close to the XRT 
position (GCN 4226), also visible in DSS and other surveys.  In our 
imaging, the galaxy is observed to be a barred spiral of type SBa.  The 
presence of a faint tidal bridge indicates that the galaxy has recently 
interacted with a neighboring galaxy 60" to the north.

Using 25 X-ray sources and 51 DSS sources in the field, we calculate an 
improved XRT position of RA = 23:01:50.21, dec = +38:40:46.0 (with a 90% 
uncertainty radius of 2.2", including an estimate of the systematic 
error.)  This position lies inside an outer spiral arm of the galaxy, 
identifying it as the GRB host.  In addition, both our position and the 
original XRT error circle include a faint optical point-like source at 
RA = 23:01:50.30, dec = +38:40:46.7 (0.5" uncertainty).

We obtained spectroscopy of the host galaxy and optical source the 
following night.  The galaxy is observed to be at a redshift of z = 
0.080, and lacks any strong emission features.  Spectroscopy of the 
point source inside the XRT error circle shows no evidence for a 
nebular-phase supernova; however, there are emission lines indicating 
active star formation in this region.

The offset of the position of the optical source to the galaxy center is 
14.1", or 20.9 kpc at the observed redshift.  This is the largest offset 
yet seen in a long GRB in angular or physical scale.  Using the 15-150 
keV gamma-ray fluence (GCN 4237), E_iso for this GRB is 4.1 x 10^48 
ergs, comparable to that observed for GRB980425 but less than that of 
any other GRB.  GRB980425/SN1998bw was also seen in a spiral galaxy at 
large offset, and also lacked a detected optical afterglow.

Given the low redshift of this system, inspection of archival images for 
the presence or absence of supernova emission is strongly encouraged. 
We also encourage radio and X-ray observations to look for a late-time 
afterglow or supernova.

Additional information on this GRB, including an image of the field, is 
available at:

http://lyra.berkeley.edu/~dperley/051109b/host.html