TITLE: GCN GRB OBSERVATION REPORT NUMBER: 4408 SUBJECT: GRB 051227: Magellan r'-band Imaging/Astrometry DATE: 05/12/28 10:30:02 GMT FROM: Josh Bloom at UC Berkeley 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/