TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2782 SUBJECT: GRB041006: Optical photometry and spectroscopy at TNG DATE: 04/10/07 17:55:28 GMT FROM: Angelo Antonelli at Obs. Astro. di Roma D. Fugazza, F. Fiore, S. Covino, L.A. Antonelli, P. D'Avanzo, F. Cocchia, D. Malesani, E. Pian, L. Stella (on behalf of the Italian CIBO collaboration); V. Lorenzi and G. Tessicini (INAF-TNG), report: "We observed the optical afterglow (Da Costa, Noel & Price, GCN 2765; Maeno et al. GCN 2772) of the X-ray rich GRB 041006 (Galassi et al., GCN 2770) with the Italian 3.6m telescope TNG at the Canary Islands. Both photometric and spectroscopic observations were carried out using DOLORES under good observing conditions (seeing of ~1"). From two R-band images (2x120 s), we obtain the following magnitude on Oct 7.07 UT (0.557 days after the GRB): R = 20.76 +- 0.04. This value is based on a calibration with Landolt standard stars. We list below magnitudes we obtain for the two stars close to the optical transient: star RA Dec R mag err - --------------------------------------------------------- U0900_00213234 00:54:47.045 +01:14:05.60 17.75 +- 0.04 U0900_00213423 00:54:50.131 +01:12:10.33 18.90 +- 0.04 Note as the above magnitudes are fainter than those given by USNO, by about 0.4 mag. This may indicate a non photometric night, or that the USNO catalog is not particularly accurate in this region of the sky. Starting on Oct 7.10 UT, a low resolution (R ~ 1000) spectrum of the transient was also obtained. Observations consisted of three exposures, for a total exposure time of 1 hour, and cover the full spectral range 3800-8000 A. The observed spectrum has a relatively low signal-to-noise and does not show any strong obvious emission line. The most prominent absorption feature is at 4800 A. If this feature is identified with the blend of the MgII lambda2803,2796 doublet, a redshift of z=0.712 is implied. If this is the case, the MgI lambda2852, FeII lambda2382,2374 and FeII lambda2344 transitions might coincide with other absorption features in the spectrum, although each single line is not statistically significant. At the estimated redshift the isotropic-equivalent gamma-ray energy of the burst is E = 1.5x10^52 erg (2-400 keV), adopting the fluence reported by Galassi et al. (GCN 2770), We are particularly grateful to the TNG staff for their remarkable support to these observations." This message is citeable. ------------------------------------------------- This mail sent through IMP: http://horde.org/imp/