Implications for the Cosmic Reionization from the Optical Afterglow Spectrum of the Gamma-Ray Burst 050904 at z = 6.3

Details Implications for the Cosmic Reionization from the Optical Afterglow Spectrum of the Gamma-Ray Burst 050904 at z = 6.3 Implications for the Cosmic Reionization from the Optical Afterglow Spectrum of the Gamma-Ray Burst 050904 at z = 6.3

Aug 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006iaujd...7e..19t&link_type=abstract

The Universe at z > 6, 26th meeting of the IAU, Joint Discussion 7, 17-18 August 2006, Prague, Czech Republic, JD07, #19

Physics

Scientific paper

The gamma-ray burst (GRB) 050904 at z = 6.3 provides the first opportunity of probing the intergalactic medium (IGM) by GRBs at the epoch of the reionization. Here we present a spectral modeling analysis of the optical afterglow spectrum taken by the Subaru Telescope, aiming to constrain the reionization history. The spectrum shows a clear damping wing at wavelengths redward of the Lyman break, and the wing shape can be fit either by a damped Ly alpha system with a column density of log (N_HI/cm^-2) ~ 21.6 at a redshift close to the detected metal absorption lines (z[metal ]= 6.295), or by almost neutral IGM extending to a slightly higher redshift of z[IGM,u] ~ 6.36. In the latter case, the difference from z[metal] may be explained by acceleration of metal absorbing shells by the activities of the GRB or its progenitor. However, we exclude this possibility by using the light transmission feature around the Ly beta resonance, leading to a firm upper limit of z[IGM,u] < 6.314. We then show an evidence that the IGM was largely ionized already at z=6.3, with the best-fit neutral fraction of IGM, x[HI] = 0.0, and an upper limit of x[HI ]< 0.17 (0.60) at 68 (95) % C.L. This is the first direct and quantitative upper limit on x[HI] at z > 6. Various systematic uncertainties are examined, but none of them appears large enough to change this conclusion. Implications for future reionization studies by using GRBs will also be presented.

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