Photometric redshifts for gamma-ray burst afterglows from GROND and Swift/UVOT

Aims. We present a framework to obtain photometric redshifts (photo-zs) for gamma-ray burst afterglows. Using multi-band photometry from GROND and Swift/UVOT, photo-zs are derived for five GRBs for which spectroscopic redshifts are not available. Methods. We use UV/optical/NIR data and synthetic photometry based on afterglow observations and theory to derive the photometric redshifts of GRBs and their accuracy. Taking into account the afterglow synchrotron emission properties, we investigate the application of photometry to derive redshifts in a theoretical range between z ∼ 1t oz ∼ 12. Results. The measurement of photo-zs for GRB afterglows provides a quick, robust and reliable determination of the distance scale to the burst, particularly in those cases where spectroscopic observations in the optical/NIR range cannot be obtained. Given a sufficiently bright and mildly reddened afterglow, the relative photo-z accuracy η =Δ z/(1 + z) is better than 10% between z = 1. 5a nd z ∼ 7 and better than 5% between z = 2a ndz = 6. We detail the approach on 5 sources without spectroscopic redshifts observed with