TITLE: GCN CIRCULAR NUMBER: 1907 SUBJECT: GRB 030227: fading of candidate optical afterglow 'S1-A' DATE: 03/02/28 10:02:17 GMT FROM: Alicia Soderberg at Caltech A. M. Soderberg, D. Fox, S. Kulkarni, and E. Berger (Caltech) report on behalf of a larger GRB collaboration: "We have imaged the field of GRB 030227 with ESI on Keck II beginning at Feb 28.242 UT in various optical bands. In the R-band image, we find two sources in the 3-arcminute (radius) error circle of the XMM source 'S1' (Loiseau et al. GCN 1901), hereafter S1-A and S1-B: 'S1-A' 04:57:33.05 +20:29:04.9 (J2000) 'S1-B' 04:57:32.80 +20:29:06.2 (J2000) The positions are accurate to 0.3" and are based on 19 stars from the GSC-II catalog. The source S1-A is clearly seen in the Isaac Newton Telescope (INT) R-band image presented by Castro-Tirado et al. (GCN 1904). We find a magnitude of R ~ 23.5 for S1-A and R ~ 25.0 for S1-B relative to the above set of GSC-II stars. Comparison with Castro-Tirado et al. (GCN 1904) indicates that the source has faded by ~1.0 magnitudes since Feb. 27.872 UT. This fading is visually confirmed by comparing our image (see URL below) with the INT image. We note that the candidate of Ofek and Gal-Yam (GCN 1903) may also be fading; with the same calibration we measure R ~ 22.1 for their source and R ~ 18.1 for their star A. Given the coincidence of S1-A with the X-ray source and its fading behavior we suggest that it is the optical afterglow of GRB 030227. Our image can be found at: http://www.astro.caltech.edu/~ams/grb030227/ Analysis of other candidates is in progress."