GCN Circular 2782

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
Not found
 
 
; 
Maeno et al. GCN 2772
Not found
 
 
) of the X-ray rich GRB 041006 (Galassi et al., 
GCN 2770
Not found
 
 
) 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
Not found
 
 
), 

We are particularly grateful to the TNG staff for their remarkable 
support to these observations."

This message is citeable.



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