TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  5508
SUBJECT: GRB 060904B: TAROT optical counterpart observations
DATE:    06/09/04 03:35:01 GMT
FROM:    Alain Klotz at CESR-CNRS  <Alain.Klotz@free.fr>

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

We imaged the field of GRB 060904B detected by SWIFT
(trigger 228006) with the TAROT robotic telescope (D=25cm)
located at the Calern observatory, France.

First image was acquired 23.1s after the GRB trigger
(10.4s after the notice). The field elevation increased from
37 degrees above horizon and weather conditions
were excellents. Images were taken during the
prompt phase.

Date of trigger : t0 = 2006-09-04T02:31:04.224

First image is a 60.0s exposure trailed from t0+23.1s
to t0+83.1s. The OT noticed by Rykoff et al.
(GCNC5504) is visible as a short flash of
magnitude R=15.8 during few seconds near t0+50s.

Second image is 30.0s exposure. OT is marginally
detected at R~18.5.

In the following images the OT becomes brighter.
A first visual inspection seems to show that
the peak of brightness (R~17) was reached
near t0+~400s.

The OT was still well detected on individual
images at t0+40min (R~17.7).

OT position (+/- 2 arcsec):

RA(J2000.0) = 03h 52m 50.62s
DEC(J2000.0) -00d 43' 29.4"

Magnitudes were estimated with the nearby USNO-B1 stars
and are not corrected for galactic dust extinction.

N.B. Galactic coordinates are lon=189.4387 lat=-39.1315
and the galactic extinction in R band is 0.1 magnitudes
estimated from D. Schlegel et al. 1998ApJ...500..525S.

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