GCN Circular 20580

A. Klotz, D. Turpin (IRAP-CNRS-OMP),
D. Macpherson (UWA/ICRAR), D. Coward (UWA),
M. Boer, R. Laugier (UNS-CNRS-OCA),
Gendre B. (UVI - Etelman Obs.),
A. Williams (PO-UWA), R. Martin (PO-UWA)
report:

We imaged the field of GRB 170202A detected by SWIFT
(trigger 736407) with the Zadko robotic telescope (D=100cm)
located at the observatory - Gingin, Australia.

The observations started 58s after the GRB trigger
(42s after the notice). The elevation of the field decreased from
51 degrees above horizon and weather conditions
were very good.

The first image is trailed with a duration of 60.0s
(see the description in Klotz et al., 2006, A&A 451, L39).
We detect the optical transcient described by Racusin et
al. (GCNC 20575), Xin et al. (GCNC 20576) and Lipunov et
al. (GCNC 20578). We calibrated magnitudes using the same
reference star than Xin et al. (GCNC 20576).

The afterglow increases slowly and continuously from 73s
(i.e. the limiting magnitude R~19) until 400s (R=16.5).
After 550s the flux start to decrease with a standard
temporal decay alpha=-1 until 5000s (R=19).

Extrapolation with this rate gives R=22.3 at t0+1day.
So we encourage to perform spectrometry.

N.B. Zadko Magnitudes are not corrected for galactic dust extinction.