TITLE:   GCN CIRCULAR
NUMBER:  2833
SUBJECT: GRB 040924:  Afterglow Color
DATE:    04/11/08 17:34:49 GMT
FROM:    Matt Bayliss at UNC,Chapel Hill  <mbayliss@physics.unc.edu>

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.