TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3171
SUBJECT: GRB050401: TNG R-band observation
DATE:    05/04/02 11:36:52 GMT
FROM:    Nicola Masetti at IASF,CNR,Bologna  <masetti@bo.iasf.cnr.it>

P. D'Avanzo, D. Fugazza (INAF-OAB), N. Masetti (INAF-IASF, Bologna) and
M. Pedani (TNG), on behalf of a larger collaboration, report:


"We have obtained R-band photometry of the optical afterglow (McNaught &
Price, GCN #3163; Rykoff et al., GCN #3165) of GRB050401 (Angelini et al.,
GCN #3161; Barbier et al., GCN #3162) using DOLoRes at TNG.
The observations, for a total exposure time of 30 minutes, were performed
under moderate seeing conditions (around 1.2 arcsec).

A preliminary analysis of the images allowed us to derive the following
photometry for the OT, calibrated after observing the SA104 Landolt
standard field:

R = 23.18 +/- 0.10  on 2005 April 2.072 UT, 11.39 hours after the burst

Using this OT magnitude and those reported in GCNs #3163 and #3165, and 
assuming a canonical power-law decay, we obtain for the OT a decay index 
alpha ~ 1, quite typical of GRB optical afterglows.

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

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