GCN Circular 3720

Klotz, A. (CESR-OMP), Boer M. (OHP), and Atteia, J.L. (LAT-OMP) report:

We imaged the entire field of GRB 050730 detected by SWIFT (Holland et al. GCNC
3704) with the TAROT robotic telescope (D=25cm) located at the Calern
observatory, France. Observations started 7.5 seconds after the GCN notice (and
66.3 s after the GRB). The field had an elevation of
29 degrees above horizon at the begining of the observations and then decreased.

The first three images taken between 2005-07-30T19:59:29.53 (GRB +
66.3s) and 2005-07-30T20:00:27.38 (GRB + 124.2s) were co-added. The afterglow is
perhaps detected very marginally. The magnitude is estimated to be
R=15.5 +/- 0.4 comparing with the USNO-B1 nearby stars. This magnitude should be
rather considered as an upper limit.

A second set of three images taken between 2005-07-30T20:00:33.60 (GRB +
130s) and 2005-07-30T20:01:46.25 (GRB + 203s) we co added.
The afterglow is not detected, suggesting R>15.9 comparing with the
USNO-B1 nearby stars.

Co addition of the six first images (GRB + 66.3s to 203s) shows the afterglow
measured to be R=16.24 +/- 0.41.

Later images were taken with longer exposure times and were co added to measure
the afterglow magnitude with a reasonable accuracy:

t=GRB + 26.79 min  R=17.00 +/- 0.41
t=GRB + 37.61 min  R=17.30 +/- 0.44
t=GRB + 48.61 min  R=17.34 +/- 0.37
t=GRB + 61.20 min  R=17.92 +/- 0.42
t=GRB + 78.47 min  R=17.54 +/- 0.39
t=GRB + 97.40 min  R=18.74 +/- 0.53

This later series gives a decay index of 0.89 +/- 0.38.

An extrapolation towards the early measurement shows that a break or a
re-brightening should occured considering our R=16.24 measurement at t=GRB +
2.31 min.

Further informations (light curve and images) on:
http://www.cesr.fr/~klotz/grb050730

This message can be cited.


--
Michel Boer
Observatoire de Haute Provence
F - 04870 Saint Michel l'Observatoire
04 92 70 64 59 - Michel.Boer@oamp.fr