TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3921
SUBJECT: GRB 050904: NIR object inside the XRT error box
DATE:    05/09/05 11:38:11 GMT
FROM:    Paolo D'Avanzo at INAF-OAB  <davanzo@merate.mi.astro.it>

P. D'Avanzo (INAF/OAB), L. A. Antonelli (INAF/OAR), S. Covino
(INAF/OAB), D. Malesani (SISSA), G. Tagliaferri, G. Chincarini
(INAF/OAB) on behalf of the MISTICI collaboration report:  

We observed the field of GRB 050904 (Cummings et al., GCN 3910), in the
J band with the 3.6m TNG telescope (equipped with NICS) and with the ESO
VLT-UT1 (equipped with ISAAC). We don't detect the NIR source at the
position indicated by Haislip et al. (R.A. 00:54:51.3, Dec. 14:05:09.7,
GCN 3913, 3919) and by Klotz et al. (GCN 3917) to a limit J > 22
(5sigma). We also note that this position is ~ 6 arcsec out from the XRT
error box (Mineo et al., GCN 3920) and ~ 11" away from its center.

We detect however a single fading source inside the XRT error box, which
is likely the NIR counterpart of GRB 050904. Its coordinates are:

  R.A.(J2000) = 00:54:50.8
  Dec.(J2000) = +14:05:10.0

The detected source faded by 0.30 +/- 0.15 mag between the two epochs
(24.7 and 26 h after the burst, respectively). That source has a J
magnitude which is consistent with the one expected from the source
proposed by Haislip et al. with a decay slope of t^-1,


A finding chart is available at the following URL:
http://www.sissa.it/~malesani/GRB/050904/GRB050904_finder.jpg

We thank the TNG and ESO staff, in particular Marco Pedani, Poshak
Gandhi and Nancy Ageorges for carefully performing our observations and
quickly providing the data.

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