The afterglow and the host galaxy of the dark burst GRB 970828

GRB 970828 was the first well-localized gamma -ray burst (GRB) X-ray afterg low for which no optical afterglow was found despite a prompt, deep search down to R-lim similar to 24.5 mag. We report the discovery of a short-lived radio flare within the X-ray localization error circle of this burst. Such radio flares are seen in about 25% of GRB afterglows, and their origin is not well understood. The precise radio position enabled us to identify the likely host galaxy of this burst and to measure its redshift, z = 0.9578. T he host appears to be an interacting/merging system. Under the assumption t hat the X-ray afterglow is mainly due to synchrotron mechanism, we infer th e optical afterglow flux. The observed upper limits to the optical flux are easily explained by invoking an intervening dusty cloud within the host ga laxy. These observations support the idea that GRBs with no detectable opti cal afterglows, or "dark GRBs," can be due to dust extinction within the ho st galaxies. The census of dark GRBs can then be used to constrain the frac tion of the obscured star formation in the universe. We argue that the exis ting data already indicate that the obscured star formation rate is no more than one-half of that seen at UV and optical wavelengths.