DETECTION OF GRB 060927 AT z = 5.47: IMPLICATIONS FOR THE USE OF GAMMA-RAY BURSTS AS PROBES OF THE END OF THE DARK AGES

摘要

We report on follow-up observations of the gamma-ray burst GRB 060927 using the robotic ROTSE-IIIa telescope and a suite of larger aperture ground-based telescopes. An optical afterglow was detected 20 s after the burst, the earliest rest-frame detection of optical emission from any GRB. Spectroscopy performed with the VLT about 13 hr after the trigger shows a continuum break at λ ≈ 8070 A, produced by neutral hydrogen absorption at z ≈ 5.6. We also detect an absorption line at 8158 A, which we interpret as Si Ⅱ λ1260 at z = 5.467. Hence, GRB 060927 is the second most distant GRB with a spectroscopically measured redshift. The shape of the red wing of the spectral break can be fitted by a damped Lyα profile with a column density with log (N_H /cm~(-2)) = 22.50 ± 0.15. We discuss the implications of this work for the use of GRBs as probes of the end of the dark ages and draw three main conclusions: (1) GRB afterglows originating from z (approx ＞) 6 should be relatively easy to detect from the ground, but rapid near-infrared monitoring is necessary to ensure that they are found; (2) the presence of large H Ⅱ column densities in some GRB host galaxies at z ＞ 5 makes the use of GRBs to probe the reionization epoch via spectroscopy of the red damping wing challenging; and (3) GRBs appear crucial to locate typical star-forming galaxies at z ＞ 5, and therefore the type of galaxies responsible for the reionization of the universe.