TITLE: GCN CIRCULAR NUMBER: 13739 SUBJECT: GRB 120909A: TAROT La Silla observatory optical observations DATE: 12/09/10 09:07:00 GMT FROM: Alain Klotz at CESR-CNRS 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). This message may be cited.