{
  "bibcode": "2004GCN..2913....1M",
  "body": "D. Malesani, S. Covino, P. D'Avanzo, G. Tagliaferri, G. Chincarini, L.A. \nAntonelli, S. Campana, M. Della Valle, F. Fiore, L. Stella, and F.M. \nZerbi report on behalf of the MISTICI collaboration:\n\nWe observed again the field of the Swift GRB 041223 (Tueller et al., GCN \n2898; Burrows et al., GCN 2901), using the ESO-VLT at Paranal. Optical \nand NIR observations were performed under excellent conditions (seeing \n0.4\") during the night of 2004 Dec 24 (~1.5 days after the GRB). We \nclearly detect the candidate afterglow reported by Berger et al. (GCN \n2902), for which we measure J = 20.43 +- 0.05 on 2004 Dec 25.07. The \nobject is pointlike at the resolution of the VLT images, and clearly \nfaded since our previous observation (Malesani et al., GCN 2903), \nconfirming that this is indeed the afterglow of GRB 041223. The decay \nindex is alpha = 1.6 +- 0.1 (F = K*t^-alpha).\n\nThe photometric SED derived from our optical and NIR observations is \nwell represented by a hard powerlaw with a spectral index beta = 0.38 +- \n0.05 (F = K * nu^-beta), after correcting for Galactic extinction (A_V = \n0.394 mag). Such hard values are not common for afterglows at these \nstages (even without any reddening correction, the spectrum is still \nhard with beta = 0.65). We note that this value is very similar to the \nX-ray spectral index ~6 hours after the GRB reported by Burrows et al. \n(GCN 2901), namely alpha = 0.43 +- 0.09.\n\nThis message can be cited.",
  "circularId": 2913,
  "createdOn": 1104090178000,
  "email": "malesani@sissa.it",
  "subject": "GRB041223: decay of the afterglow",
  "submitter": "Daniele Malesani at SISSA-ISAS,Trieste,Italy  <malesani@sissa.it>",
  "eventId": "GRB 041223"
}