TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2833 SUBJECT: GRB 040924: Afterglow Color DATE: 04/11/08 17:34:49 GMT FROM: Matt Bayliss at UNC,Chapel Hill J. Silvey, D. Allen, M. Bayliss, M. Nysewander, T. Tilleman, A. Henden, M. Leake, A. Homewood, R. Canterna, D. Reichart, D. H. Hartmann, and M. Schwartz report on behalf of the U. Wyoming, U. North Carolina, USNO, and SARA GRB teams of the FUN GRB collaboration: We observed the afterglow location (Fox & Moon, GCN 2734) of the short/soft GRB 040924 (Fenimore et al., GCN 2735) in VRcIcH beginning 16.2 hours after the burst. Using the field calibrations of Henden (GCN 2811) and 2MASS, we report the following magnitudes: Start Mean Time Filter Exposure Magnitude Telescope Date Since GRB Time (sec) (1) (hours) x Exposures Sep 25.171 16.44 Rc 300 x 4 >18.9 0.9m SARA Sep 25.191 16.89 Ic 300 x 4 >20.6 0.9m SARA Sep 25.209 17.31 V 300 x 4 >19.6 0.9m SARA Sep 25.228 18.32 H 800 x 4 >19.5 1.55m NOFS Sep 25.328 21.18 Ic 300 x 24 >21.7 0.8m TII(2) Sep 25.391 21.86 Rc 300 x 3 22.68+/-0.07 2.3m WIRO Sep 25.395 21.96 V 300 x 3 23.06+/-0.07 2.3m WIRO (1) Limiting magnitudes are 2 sigma. (2) Tenagra II Compared to the R observations of Fox (GCN 2741) and Pavlenko et al. (GCN 2753), our Rc observation is generally consistent with a temporal index of approx. -1. Assuming a temporal index of -1.16 (Khamitov et al., GCN 2749) and using this index to scale our observations and the K' observation of Terada et al. (GCN 2750) to 21.9 hours after the burst, and correcting for Galactic extinction along the line of sight assuming that R_V = 3.1, yields a spectral index of -0.61 +/- 0.08: www.physics.unc.edu/~mbayliss/grbdata/grb040924spec.eps If source-frame extinction is negligible, this is consistent with either a constant-density or wind-swept medium with the cooling break blueward of the observed bands. It is inconsistent with either a constant-density or wind-swept medium with the cooling break redward of the observed bands (e.g., Sari, Piran & Narayan 1998, ApJ, 497, L17; Chevalier & Li 2000; ApJ, 536, 195). However, if source-frame extinction is non-negligible this is only consistent with a wind-swept medium with the cooling break blueward of the observed bands.