GCN Circular 13739

Klotz A. (IRAP-CNRS-OMP), Gendre B. (ASDC/INAF-OAR),
Boer M. (UNS-CNRS-OCA), Atteia J.L. (IRAP-CNRS-OMP) report:

We imaged the field of GRB 120909A detected by SWIFT
(trigger 533060) with the TAROT robotic telescope (D=25cm)
located at the European Southern Observatory,
La Silla observatory, Chile.

The observations started 40s after the GRB trigger
(14s after the notice). The elevation of the field decreased
from 54 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).
We detect the optical transient discovered by
Haislip et al. (GCNC 13728).

Relative photometric analysis is done using R=14.37
for the star NOMAD1 0305-0869420 (R.A.=275.7585731
Decl=-59.4476147 J2000).

The careful analysis of the trail is compatible with a
constant magnitude of R=17.1 from T+40s to T+100s.
There is not optical peak during the gamma peak
at T+48s reported by Ukwatta et al. (GCNC 13733).

After the trailed image, a technical problem caused the
lost of images until T+439s. Following images have been
acquired using the classical diurnal drift.

Summary of the first observations is:

Start  Stop  Rmag  Error
   40s  100s  17.1    0.1 (continuous photometry)
  439s  529s  16.5    0.1
  539s  629s  18.9    0.1
  640s  729s  19.1    0.2
  744s  834s  19.3    0.2
  845s  935s  19.7    0.2

Magnitudes are not corrected for galactic dust extinction
(which should be about 0.4 magnitude in the R band according
to D. Schlegel et al. 1998ApJ...500..525S).

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