TITLE: GCN CIRCULAR NUMBER: 12242 SUBJECT: GRB110731A: MOA optical observations of the steeply fading afterglow DATE: 11/08/03 11:49:40 GMT FROM: Akihiko Fukui at Nagoya U/MOA P. J. Tristram (MJUO), A. Fukui (Nagoya U.), K. Ohnishi (Nagano NCT) and T. Sako (Nagoya U.) report on behalf of the MOA collaboration. We began prompt observations of the afterglow of GRB110731A (Oates et al., GCN 12215) at July 31 11:12:47 UT responding to the Swift trigger 458448 (3.3 min after the trigger) using the 61 cm B&C telescope at the Mt John University Observatory in New Zealand. We obtained a series of I and V band images with 60 sec exposure times followed by some 120 sec exposures until 12:56 UT (105 min after the trigger). The total number of I and V images are 39 and 35, respectively. Although the first image had a poor psf, the following images had well-shaped psf's and we confirmed the steeply fading afterglow, which is consistent with the previous reports (e.g. Bersier et al. GCN 12216), on these images in both the I and V bands. The afterglow is located within the error circle reported by Oates et al. GCN 12222. The magnitude of the afterglow on the second and on the last images in the I band obtained by psf-fitting photometry with the DoPHOT package (Schechter et al. 1993) and calibrated to the USNO-B1.0 N magnitude (similar to I magnitude) follow. ------------------------------------------ Filter mid_JD-2450000 exptime[sec] dT[min] Nmag error I 5773.96831 60 4.86 14.75 0.20 I 5774.03807 120 105.31 20.2 2.7 ------------------------------------------ Here dT denotes the difference between the mid_JD (mid time of exposure) and the trigger time. The large error in the magnitude of the last image resulted from the calibration relying on extrapolation.