GCN Circular 22671

Klotz A., Noysena.K., Atteia J.L. (CNRS-OMP-IRAP),
Boer, M., Eymar, L. (CNRS-ARTEMIS),
Gendre B. (UVI - Etelman Obs.) report:

We imaged the field of GRB 180418A detected by SWIFT
(trigger 826428) with the TAROT robotic telescope (D=25cm)
located at the European Southern Observatory,
La Silla observatory, Chile.

The observations started 28s after the GRB trigger
(15s after the notice). The elevation of the field decreased from
6 degrees above horizon and weather conditions were good.

The first image is trailed with a duration of 60.0s
(see the description in Klotz et al., 2006, A&A 451, L39).
Despite the very elevation we can measure the very
early optical light variation of the OT reported by
Zheng & Filippenko (GCNC 22647). We used NOMAD-1 1149-0186465
as reference star r(AB)=15.23 (according SDSS DR9).
Preliminary analysis gives:

start end  r(AB) 1sig
(sec) (sec)
   28   36  14.30 0.04
   36   43  14.24 0.03
   43   51  14.40 0.03
   51   58  14.61 0.04
   58   66  14.98 0.05
   66   73  15.42 0.06
   73   81  15.52 0.09
   81   88  15.83 0.15

The following images are taken in tracking mode.
We used the same reference star for PSF fitting:

start end  r(AB) 1sig
(sec) (sec)
  100  130  15.96 0.06
  141  171  16.50 0.11
  181  211  16.58 0.13
  221  251  17.10 0.31
  262  292  16.90 0.25
  302  392  17.00 0.29

Magnitudes are not corrected for galactic dust extinction.

We note that the maximum brightness of the OT
seems to occured at 40s r(AB)=14.24 followed by
a decay of about alpha=2.7 until 70s. Then the decay
is compatible with alpha=1.1 until 6 min (end of TAROT
observations because the elevation was too low).