GRB 080913 AT REDSHIFT 6.7

Abstract

We report on the detection by Swift of GRB 080913, and subsequent\noptical/near-infrared follow-up observations by GROND which led to the\ndiscovery of its optical/NIR afterglow and the recognition of its high-z nature\nvia the detection of a spectral break between the i' and z' bands. Spectroscopy\nobtained at the ESO-VLT revealed a continuum extending down to lambda = 9400 A,\nand zero flux for 7500 A < lambda<9400 A, which we interpret as the onset of a\nGunn-Peterson trough at z=6.695+-0.025 (95.5% conf. level), making GRB 080913\nthe highest redshift GRB to date, and more distant than the highest-redshift\nQSO. We note that many redshift indicators which are based on promptly\navailable burst or afterglow properties have failed for GRB 080913. We report\non our follow-up campaign and compare the properties of GRB 080913 with bursts\nat lower redshift. In particular, since the afterglow of this burst is fainter\nthan typical for GRBs, we show that 2m-class telescopes can identify most\nhigh-redshift GRBs.\n