TITLE:   GCN CIRCULAR
NUMBER:  10200
SUBJECT: GRB 091127: TAROT La Silla observatory optical decay
DATE:    09/11/28 08:22:39 GMT
FROM:    Alain Klotz at CESR-CNRS  <Alain.Klotz@free.fr>

Klotz A. (CESR-OMP), Gendre B. (IASF),
Boer M. (OHP-OAMP), Atteia J.L. (LATT-OMP) report:

We imaged the field of GRB 091127 detected by Swift
(trigger 377179) with the TAROT robotic telescope (D=25cm)
located at the European Southern Observatory,
La Silla observatory, Chile.

The observations started 1 hour after the GRB trigger
(event occured during dusk). The elevation of the field
increased from 58 degrees above horizon and weather conditions
were good.

We detected the optical couterpart (OT) mentioned by
Smith et al. 2009 (GCNC 10192). As the OT is bright,
we used only R filtered images. We used the reference
star at coordinates 02:26:15.06 -18:54:57.3 R=15.82,
(V-R)=0.24 which is the closest star with (V-R) close
to that of the GRB (V-R~0.1 according to GROND observations
by Updike et al. GCNC 10195). The star at coodinates
02:26:21.07 -18:57:18.9 R=14.02 lies at 20 arcsec of the OT.
We substract this star before photometry:

-----------------------------------
  Tstart    Tstop  Rmag
  (min)     (min)
-----------------------------------
   69.12 - 82.19  16.4 +/- 0.2
  106.59 - 119.67 16.7 +/- 0.3
  144.33 - 157.41 16.7 +/- 0.3
  182.09 - 222.83 17.0 +/- 0.2
  229.91 - 346.22 17.1 +/- 0.2
  367.71 - 444.61 17.4 +/- 0.2
-----------------------------------

The decay is very low since the OT decreased
of only 1 mag in 5 hours !