The 2175 Å dust feature in a gamma-ray burst afterglow at redshift 2.45

We present optical and near-infrared photometry of the afterglow of the long gamma-ray burst (GRB) 070802 at redshift 2.45 obtained with the ESO/MPI 2.2 m telescope equipped with the multichannel imager GROND. Follow-up observations in g'r'i'z' and JHK(S) bands started at similar to 17 minutes and extended up to 28 hr postburst. We find an increase in the brightness of the afterglow at early times, which can be explained by the superposition of reverse and forward shock (FS) emission or the onset of the afterglow FS. In addition, we detect a strong broadband absorption feature in the i' band, which we interpret as extinction from the redshifted 2175 angstrom bump in the GRB host galaxy. This is one of the first and clearest detections of the 2175 angstrom feature at high redshift. It is strong evidence for a carbon-rich environment, indicating that Milky Way-or Large Magellanic Cloud-like dust was already formed in substantial amounts in a galaxy at z = 2.45.