GCN Circular 8771

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.