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
Ruiz-Velasco, A. E. and Swan, H. F. and Troja, E. and Malesani, D and Fynbo, J. P. U. and Starling, R. L. C. and Xu, D. and Aharonian, F. and Akerlof, C. W. and Andersen, M. I. and Ashley, M.C.B. and Barthelmy, S.D. and Bersier, D. and Cerón, M. Castro and Castro-Tirado, A. J. and Gehrels, N. and Göğüş, Ersin and Gorosabel, J. and Guidorzi, C. and Güver, T. and Hjorth, J. and Horns, D. and Huang, K. Y. and Jakobsson, P. and Jensen, B. L. and Kızıloğlu, Ü. and Kouveliotou, C. and Krimm, H. A. and Ledoux, C. and Levan, A. J. and Marsh, T. and McKay, T. and Melandri, A. and Milvang-Jensen, B. and Mundell, C. G. and O’Brien, P. T. and Özel, M. and Phillips, A. and Quimby, R. and Rowell, G. and Rujopakarn, W. and Rykoff, E. S. and Schaefer, B. E. and Sollerman, J. and Tanvir, N. R. and Thöne, C. C. and Urata, Y. and Vestrand, W. T. and Vreeswijk, P. M. and Watson, D. and Wheeler, J. C. and Wijers, R. A. M. J. and Wren, J. and Yost, S. A. and Yuan, F. and Zhai, M. and Zheng, W. K. (2007) 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. The Astrophysical Journal, 669 (issue 1 part 1). pp. 1-9. ISSN 0004-637
We report on follow-up observations of the GRB 060927 using the 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 hours after the trigger shows a continuum break at lambda ~ 8070 A produced by neutral hydrogen absorption at z~5.6. We also detect an absorption line at 8158 A which we interpret as SiII 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 Lyalpha profile with a column density with log(N_HI/cm^-2) ~ 22.5. 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: i) GRB afterglows originating from z>6 should be relatively easy to detect from the ground, but rapid NIR monitoring is necessary to ensure that they are found; ii) The presence of large HI column densities in some GRBs host galaxies at z>5 makes the use of GRBs to probe the reionization epoch via spectroscopy of the red damping wing challenging; iii) 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.
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