TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2168 SUBJECT: Optical Observations of GRB 030418 with Magellan DATE: 03/04/25 01:11:43 GMT FROM: George Ricker at MIT Optical Observations of GRB 030418 with Magellan A. Dullighan, N. R. Butler, G. R. Ricker, J. L. Elliot, K. Clancy (MIT), and D. Osip (Las Campanas Observatory) report: On 2003 April 21.01 UT, we observed the optical afterglow (Price et al; GCN2148) of the HETE-localized GRB 030418 (=H2686) with the Magellan 6.5 meter Clay telescope at Las Campanas Observatory in Chile. Our observations consisted of three 300s R-Band images and one 300s I-Band image, taken with the MagIC instrument. The afterglow brightness had declined significantly since the measurement reported in GCN2148, consistent with a power law decay index = 1.3+/-0.1. The afterglow was still point-like in our images, with a FWHM of 0.5 arcsec. Our results were as follows: UT t-t0 (days) Mag 2003-04-21.02 2.60 R=23.64 +/- 0.07 2003-04-21.03 2.61 I=22.60 +/- 0.05 The magnitudes reported above are relative to those of the USNO B1 catalog star at RA = 10h 54m 32.71s, Dec = -07:02:16.4 (J2000), taken as having R=19.33 and I=18.31. Magnitude errors for the USNO star were not available, and so are not included in the errors we report here. Possible errors in the magnitudes of the USNO catalog star should be taken into account when comparing our afterglow magnitudes to other measurements. Further observations are planned. This message may be cited.