TITLE: GCN GRB OBSERVATION REPORT NUMBER: 5019 SUBJECT: GRB 060428b: Optical afterglow candidate DATE: 06/04/28 11:15:34 GMT FROM: Paul Price at IfA,UH P.A. Price (IfA, Hawaii), T. Minezaki (IoA, Tokyo), L.L. Cowie, Y. Kakazu (IfA, Hawaii) and Y. Yoshii (IoA, Tokyo) report: We have observed the XRT localisation of GRB 060428B (GCN #5017) with the robotic MAGNUM telescope. Observations were made under clouds in R and I bands starting at 2006 Apr 28.38 UTC, with interruptions due to weather. Observations consisted of 10 x 65 sec exposures in R-band. We identify a source in the combined image, which does not appear to be in the DSS 2 red plate. While a precise astrometric solution is not yet available, our estimate of the position is approximately 3 arcsec west of the USNO-B1 source 1520-0246226 (at 15:41:25.96 +62:01:30.160 J2000), or around 15:41:25.5 +62:01:30 J2000 The source is approximately R ~ 19.6 mag at the time of our observations (from comparison with the USNO-B1 catalogue), and also appears in subsequent I-band and R-band combined images. An image of the field is available at http://www.ifa.hawaii.edu/~price/grb060428b.jpg