TITLE: GCN CIRCULAR NUMBER: 8771 SUBJECT: GRB 090102: GROND observations of the optical afterglow DATE: 09/01/02 23:59:05 GMT FROM: Paulo M. J. Afonso at MPE GRB 090102: GROND observations of the optical afterglow P. Afonso, T. Kruehler (both MPE Garching), S. Klose (TLS Tautenburg), and J. Greiner (MPE Garching) report on behalf of the GROND team: GROND (Greiner et al. 2008, PASP 120, 405), mounted at the 2.2m ESO/MPI telescope at La Silla Observatory (Chile),started observations of the field of GRB 090102 (Mangano et al. 2008, GCN #8762) in g'r'i'z'JHK at 05:25 UTC, 2.50 h after the burst. Preliminary photometry yields the following r' band magnitudes of the optical afterglow (Klotz et al., GCN #8761, and also GCNs #8762, #8763, #8764, #8766 and #8770), calibrated against SDSS field stars. T_mid[s] Exp[s] AB Mag MagErr ------------------------------- 9042 66 20.58 0.05 9158 66 20.59 0.04 9265 66 20.58 0.03 9373 66 20.59 0.06 9515 115 20.59 0.04 9711 115 20.65 0.02 9903 115 20.67 0.02 10091 115 20.66 0.03 10423 375 20.70 0.02 10868 375 20.78 0.02 11320 375 20.79 0.02 11780 375 20.86 0.02 The quoted error is statistical only. There is an additional systematic error in the absolute calibration using SDSS field stars which is expected to be in the 0.05 mag range. In this time interval, the light curve is well described with a single power law of index 1.0 +/- 0.1, compatible with the value reported by Curran et al. (GCN #8770). Comparing against the early TAROT data (Klotz et al., GCN #8764), this indicates a flattening of the light curve. If the afterglow continues to decline with this power law, we predict r' band magnitudes of ~23.1 at 1 day and ~24.3 at 3 days after the burst. After correcting for a Galactic foreground extinction corresponding to a reddening of E_(B-V)=0.05 (Schlegel et al. 1998), the g' to K band SED is well described with a power law of spectral index beta = 1.0 +- 0.2.