GCN Circular 2913
Subject
GRB041223: decay of the afterglow
Date
2004-12-26T19:42:58Z (20 years ago)
From
Daniele Malesani at SISSA-ISAS,Trieste,Italy <malesani@sissa.it>
D. Malesani, S. Covino, P. D'Avanzo, G. Tagliaferri, G. Chincarini, L.A.
Antonelli, S. Campana, M. Della Valle, F. Fiore, L. Stella, and F.M.
Zerbi report on behalf of the MISTICI collaboration:
We observed again the field of the Swift GRB 041223 (Tueller et al., GCN
2898; Burrows et al., GCN 2901), using the ESO-VLT at Paranal. Optical
and NIR observations were performed under excellent conditions (seeing
0.4") during the night of 2004 Dec 24 (~1.5 days after the GRB). We
clearly detect the candidate afterglow reported by Berger et al. (GCN
2902), for which we measure J = 20.43 +- 0.05 on 2004 Dec 25.07. The
object is pointlike at the resolution of the VLT images, and clearly
faded since our previous observation (Malesani et al., GCN 2903),
confirming that this is indeed the afterglow of GRB 041223. The decay
index is alpha = 1.6 +- 0.1 (F = K*t^-alpha).
The photometric SED derived from our optical and NIR observations is
well represented by a hard powerlaw with a spectral index beta = 0.38 +-
0.05 (F = K * nu^-beta), after correcting for Galactic extinction (A_V =
0.394 mag). Such hard values are not common for afterglows at these
stages (even without any reddening correction, the spectrum is still
hard with beta = 0.65). We note that this value is very similar to the
X-ray spectral index ~6 hours after the GRB reported by Burrows et al.
(GCN 2901), namely alpha = 0.43 +- 0.09.
This message can be cited.