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*

A. E. Ruiz-Velasco; H. Swan; E. Troja; D. Malesani; J. P. U. Fynbo; R. L. C. Starling; D. Xu; F. Aharonian; C. Akerlof; M. I. Andersen; M. C. B. Ashley; S. D. Barthelmy; D. Bersier; J. M. Castro Cerón; A. J. Castro-Tirado; N. Gehrels; E. Göğüş; J. Gorosabel; C. Guidorzi; T. Güver; J. Hjorth; D. Horns; K. Y. Huang; P. Jakobsson; B. L. Jensen; Ü. Kızıloğlu; C. Kouveliotou; H. A. Krimm; C. Ledoux; A. J. Levan; T. Marsh; T. McKay; A. Melandri; B. Milvang-Jensen; C. G. Mundell; P. T. O’Brien; M. Özel; A. Phillips; R. Quimby; G. Rowell; W. Rujopakarn; E. S. Rykoff; B. E. Schaefer; J. Sollerman; N. R. Tanvir; C. C. Thöne; Y. Urata; W. T. Vestrand; P. M. Vreeswijk; D. Watson; J. C. Wheeler; R. A. M. J. Wijers; J. Wren; S. A. Yost; F. Yuan; M. Zhai; W. K. Zheng

doi:10.1086/521546

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^*

A. E. Ruiz-Velasco^1,2, H. Swan³, E. Troja^4,5,6, D. Malesani¹, J. P. U. Fynbo¹, R. L. C. Starling⁴, D. Xu¹, F. Aharonian⁷, C. Akerlof³, M. I. Andersen⁸, M. C. B. Ashley⁹, S. D. Barthelmy¹⁰, D. Bersier¹¹, J. M. Castro Cerón¹, A. J. Castro-Tirado¹², N. Gehrels¹⁰, E. Göğüş¹³, J. Gorosabel¹², C. Guidorzi^11,14, T. Güver¹⁵, J. Hjorth¹, D. Horns⁷, K. Y. Huang¹⁶, P. Jakobsson¹⁷, B. L. Jensen¹, Ü. Kızıloğlu¹⁸, C. Kouveliotou¹⁹, H. A. Krimm^10,20, C. Ledoux²¹, A. J. Levan²², T. Marsh²², T. McKay³, A. Melandri¹¹, B. Milvang-Jensen¹, C. G. Mundell¹¹, P. T. O'Brien⁴, M. Özel²³, A. Phillips⁹, R. Quimby²⁴, G. Rowell⁷, W. Rujopakarn²⁵, E. S. Rykoff³, B. E. Schaefer²⁶, J. Sollerman¹, N. R. Tanvir⁴, C. C. Thöne¹, Y. Urata²⁷, W. T. Vestrand²⁸, P. M. Vreeswijk²¹, D. Watson¹, J. C. Wheeler²⁴, R. A. M. J. Wijers²⁹, J. Wren²⁸, S. A. Yost³, F. Yuan³, M. Zhai³⁰, and W. K. Zheng³⁰

© 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A.
The Astrophysical Journal, Volume 669, Number 1 Citation A. E. Ruiz-Velasco et al 2007 ApJ 669 1 DOI 10.1086/521546

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¹ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark

² Departmento de Astronomía, Universidad de Guanajuato, Apartado Postal 144, 36000 Guanajuato, Mexico

³ University of Michigan, 2477 Randall Laboratory, 450 Church Street, Ann Arbor, MI 48109

⁴ Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK

⁵ INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica Sezione di Palermo, via Ugo La Malfa 153, 90146 Palermo, Italy

⁶ Dipartimento di Scienze Fisiche ed Astronomiche, Sezione di Astronomia, Università di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy

⁷ Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany

⁸ Astrophysikalisches Institut, 14482 Potsdam, Germany

⁹ School of Physics, Department of Astrophysics and Optics, University of New South Wales, Sydney, NSW 2052, Australia

¹⁰ NASA/Goddard Space Flight Center, Greenbelt, MD 20771

¹¹ Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD, UK

¹² Instituto de Astrofísica de Andalucía (IAA-CSIC), Apartado de Correos, 3.004, E-18.080 Granada, Spain

¹³ Sabancı University, Orhanlı-Tuzla, 34956 Istanbul, Turkey

¹⁴ INAF-Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate, Italy

¹⁵ Istanbul University Science Faculty, Department of Astronomy and Space Sciences, 34119 Istanbul, Turkey

¹⁶ Institute of Astronomy, National Central University, Ching-Li 32054, Taiwan

¹⁷ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK

¹⁸ Middle East Technical University, 06531 Ankara, Turkey

¹⁹ NASA Marshall Space Flight Center, NSSTC, VP-62, 320 Sparkman Drive, Huntsville, AL 35805

²⁰ Universities Space Research Association, 10211 Wincopan Circle, Suite 500, Columbia, MD 21044

²¹ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago 19, Chile

²² Department of Physics, University of Warwick, Coventry CV4 7AL, UK

²³ Çanakkale Onsekiz Mart Üniversitesi, Terzioıglu 17020, Çanakkale, Turkey

²⁴ Department of Astronomy, University of Texas, Austin, TX 78712

²⁵ Steward Observatory, University of Arizona, Tucson, AZ 85721

²⁶ Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803

²⁷ Department of Physics, Saitama University, Shimo-Okubo, Sakura, Saitama 338-8570, Japan

²⁸ Los Alamos National Laboratory, NIS-2, MS D436, Los Alamos, NM 87545

²⁹ Institute of Astronomy "Anton Pannekoek," University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, Netherlands

³⁰ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China

Dates

Received 2007 April 19
Accepted 2007 July 10

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Abstract

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 Å, produced by neutral hydrogen absorption at z ≈ 5.6. We also detect an absorption line at 8158 Å, 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 ≳ 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 I 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.

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Footnotes

*
Partly based on observations carried out with the ESO telescopes under programs 077.D-0661, 077.A-0667, 078.D-0416, and the large program 177.A-f0591.

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