TITLE:   GCN CIRCULAR
NUMBER:  4408
SUBJECT: GRB 051227: Magellan r'-band Imaging/Astrometry
DATE:    05/12/28 10:30:02 GMT
FROM:    Josh Bloom at UC Berkeley  <jbloom@astron.berkeley.edu>

J. S. Bloom (UC Berkeley), P. Challis (CfA), A. Garg (CfA), and R.  
Foley (UC Berkeley) report:

"Starting at 2005-12-28 06:16:19 UT, we began imaging the field of  
GRB 051227 (GCNs 4397, 4400, 4401, 4403) with the Magellan 6.5m +  
LDSS3 instrument (*). At an airmass of 2.1 the effective seeing was  
0.62 arcsec. In each five minute exposure we detect the faint VLT  
sources noted by Malesani et al. (GCNs 4404, 4407) near the XRT  
localization (Beardmore et al., GCN 4402). Photometric analysis is  
ongoing. We fit an astrometric world coordinate system using more  
than one hundred objects common between the LDSS3 imaging and SDSS  
(Cool et al. 4399) with an rms of 85 mas in each coordinate. Our  
location of the possibly transient source (Malesani et al.; GCN 4407)  
is (J2000):

          RA        08:20:58.109
          DEC  +31:55:31.89

with a statistical uncertainty of 210 mas in each coordinate from  
centroiding uncertainty. The absolute uncertainty is ~250 mas,  
assuming 100 mas error in the absolute SDSS WCS.  This is 1.89" S and  
4.28" W of an apparent galaxy (SDSS J082058.46+315533.4; r = 22.9  
mag; 08:20:58.445 +31:55:33.78). If the galaxy and the apparent  
counterpart are at the redshift of the cluster (NSC J082047+315700 as  
noted by Barbier GCN# 4397; z = 0.1753; Gal et al. 2003), then this  
offset would amount to 13.7 kpc in projection."

A comparison finding chart between tonight's imaging with SDSS can be  
found at:
    http://astro.berkeley.edu/~jbloom/grb051227-ldss.ps

This message may be cited.

(*) http://www.ociw.edu/lco/magellan/instruments/LDSS3/