GCN Circular 9773

Adria Updike (Clemson University), Arne Rau, Thomas Kruehler, Felipe 
Olivares, and Jochen Greiner (all MPE Garching) report on behalf of the 
GROND team:

We observed the field of GRB 090812 (Swift trigger 359711; Stamatikos et 
al., GCN #9768) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 
2008, PASP 120, 405) mounted at the 2.2 m ESO/MPI telescope at La Silla 
Observatory (Chile).

Due to cloud coverage, observations started only at 08:13 UT on August 12, 
2 hours and 11 minutes after the GRB trigger. They were performed at an 
average seeing of 1.1" and at an average airmass of 1.2 with light cirrus, 
and continued until twilight.

We clearly detect the afterglow in stacked images (Stamatikos et al. GCN 
#9768, Cenko GCN #9769). Based on the first 4 min of total exposures, we 
estimate preliminary magnitudes and upper limits (all in AB system) of

g' = 22.15 +- 0.15 mag,
r' = 21.50 +- 0.08 mag,
i' = 21.30 +- 0.13 mag,
z' = 21.02 +- 0.12 mag,
J = 20.57 +- 0.15 mag,
H > 19.8 mag,
K > 18.9 mag

The given magnitudes are calibrated against SDSS as well as 2MASS field 
stars and are not corrected for the expected Galactic foreground extinction 
corresponding to a reddening of E_(B-V)=0.02 mag in the direction of the 
burst (Schlegel et al. 1998).

The foreground extinction corrected spectral energy distribution is well 
fit with a pure power law and a spectral index of beta = 1.1+/-0.2 and no 
strong evidence of excess extinction.

The flux difference between the g' band magnitude and the extrapolation of 
the power law as defined by r'i'z'J is consistent with Lyman-alpha 
absorption at a redshift of 2.45 as reported by de Ugarte Postigo et al. 
(GCN #9771).