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TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2236 SUBJECT: GRB 030418: Second Epoch Optical Observations with Magellan DATE: 03/05/20 03:12:39 GMT FROM: George Ricker at MIT <grr@space.mit.edu> GRB 030418: Second Epoch Optical Observations 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 25.00 UT, we observed the optical afterglow (Price et al; GCN2148) of the HETE-localized GRB 030418 (Shirasaki et al; GCN2210) with the Magellan 6.5 meter Clay telescope at Las Campanas Observatory in Chile. Our observations consisted of two 400s R-Band images, one 400s I-Band image, and one 400s V-Band image taken with the MagIC instrument. The afterglow declined in brightness with respect to our previous measurements (Dullighan et al; GCN2168). The R band data is consistent with a revised power law decay index = 1.2 +/- 0.1, when the two data points from Price et al. (GCN2148 and GCN2150) are included. The afterglow was still point-like, but very close to the limiting magnitudes of the images. No evidence of a host galaxy is observed. We can, however, place a limit on the redshift of the afterglow of z<~5, due to the small V-R color difference we observe (i.e., no evidence for a Lyman alpha break). Our second epoch results were as follows: UT t-t0 (days) Mag 2003-04-25.00 6.58 R=24.9 +/- 0.4 2003-04-25.01 6.59 I>24 (limiting magnitude) 2003-04-25.02 6.60 V=25.1 +/- 0.5 The magnitudes reported above are referenced to the photometry reported by A. Henden (GCN2216). We have also recalibrated the magnitudes we reported in GCN 2168 to A. Henden's photometry (GCN2216). The errors in calculating relative magnitudes have now been taken into account. Our revised first epoch results are as follows: UT t-t0 (days) Mag (Original) Mag (Revised) 2003-04-21.02 2.60 R=23.64 +/- 0.07 R=23.3 +/- 0.1 2003-04-21.03 2.61 I=22.60 +/- 0.05 I=23.0 +/- 0.1 This message may be cited.
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