GCN Circular 3089

P. D'Avanzo, D. Fugazza, S. Covino (INAF-OABr), D. Malesani (SISSA), N. 
Masetti, E. Palazzi (IASF-CNR), L.A. Antonelli, G.L. Israel (INAF-OARm), 
and G. Andreuzzi (INAF-TNG), report on behalf of a larger Italian 
collaboration:

We imaged again (Fugazza et al., GCN 3078
Not found
 
 
) the field of GRB 050306 
(Markwardt et al., GCN 3071
Not found
 
 
) with the Italian TNG telescope. 
Observations were carried out under a clear sky and with a seeing of 
about 1.6", starting on 2005 Mar 9.247 UT. Seven R-band exposures were 
acquired, with an overall exposure time of 21 minutes (mean UT Mar 
9.260, i.e. 3.11 days after the burst).

Inside the revised error circle of the XRT afterglow (Perri et al., GCN 
3075
Not found
 
 
; Mangano et al., GCN 3086
Not found
 
 
), we detect several sources. After 
performing PSF photometry with DAOPhot, we find that the object at 
coordinates:

  alpha(J2000) =  18:49:13.95
  delta(J2000) = -09:09:08.6

(that is, 3.5" off the XRT centroid position) has faded by 0.54+-0.10 
mag since our previous observation, carried out ~2.1 days after the GRB 
(Fugazza et al., GCN 3078
Not found
 
 
). Assuming a powerlaw decay (F(t) = 
K*t^-delta), this corresponds to delta = 1.28+-0.24, a value typical 
among GRB afterglows at comparable epochs. We thus propose that this 
object is the optical afterglow of GRB050603. Based on calibration upon 
USNO stars, the object has R ~ 23 at the time of our first observation, 
Mar 8.25 UT.

A finding chart of the field is posted at the following URL:
http://www.sissa.it/~malesani/GRB/050306

Further observations are planned at the TNG in the following nights, but 
no spectroscopy will be possible.

We warmly acknowledge the TNG staff for carefully performing the 
observations.

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