The reanalysis of spectra of GRB 080913 to estimate the neutral fraction of the IGM at a redshift of 6.7

Abstract

Aims: We reanalyse optical spectra of the z=6.7 gamma-ray burst GRB 080913, adding hitherto unpublished spectra, in order to reassess the measurement of the neutral fraction of the IGM at high redshifts. Methods: In the data reduction we take particular care to minimise systematic errors in the sky subtraction, which are evident in the published spectrum, and compromise the analysis. The final combined spectrum has a higher S/N than the previously published spectrum by a factor of 1.3. Results: We find a single significant absorption line redward of the Ly-alpha continuum break, which we identify with the SII+SiII blend (rest wavelength of 0.1260 micron), at z=6.733. The sharp spectral break at Ly-alpha implies a comparatively low total column density of neutral hydrogen along the line of sight, log [N(HI)/cm^-2] < 20. We model the absorption with a host-galaxy DLA, surrounded by an ionised region of unknown size r, within the IGM of neutral fraction, x(HI). Despite knowing the source redshift, and the improved S/N of the spectrum, when fitting only over wavelengths redward of Ly-alpha, no useful constraints on x(HI) can be obtained. We consider the possibility of including the ionised region, blueward of Ly-alpha, in constraining the fit. For the optimistic assumption that the ionised region is transparent, tau_{GP} << 1, we find that the region is of small size r < 2 proper Mpc, and we obtain an upper limit to the neutral fraction of the IGM at z=6.7 of x(HI) < 0.73.

arXiv comment4 pages, 4 figures, A&A accepted

References in corpus (24)

• Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Likelihoods and Parameters from the WMAP data
• Cosmic-Ray Rejection by Laplacian Edge Detection
• Constraining the Evolution of the Ionizing Background and the Epoch of Reionization with z ~ 6 Quasars II: A Sample of 19 Quasars
• A Survey of z>5.7 Quasars in the Sloan Digital Sky Survey II: Discovery of Three Additional Quasars at z>6
• A glimpse of the end of the dark ages: the gamma-ray burst of 23 April 2009 at redshift 8.3
• GRB 090423 at a redshift of z~8.1
• The End of the Reionization Epoch Probed by Ly-alpha Emitters at z=6.5 in the Subaru Deep Field
• Low-Resolution Spectroscopy of Gamma-ray Burst Optical Afterglows: Biases in the Swift Sample and Characterization of the Absorbers
• Implications for the Cosmic Reionization from the Optical Afterglow Spectrum of the Gamma-Ray Burst 050904 at z = 6.3
• The Discovery of a Luminous z=5.80 Quasar from the Sloan Digital Sky Survey
• Four quasars above redshift 6 discovered by the Canada-France High-z Quasar Survey
• Studying Reionization with Ly-alpha Emitters
• GRB 080913 at redshift 6.7
• The Impact of The IGM on High-Redshift Lyman Alpha Emission Lines
• The Reionization and Galaxy Evolution Probed by z=7 Lyman Alpha Emitters
• The Detectability of High Redshift Lyman Alpha Emission Lines Prior to the Reionization of the Universe
• Ly-alpha Radiative Transfer in Cosmological Simulations and Application to a z~8 Emitter
• Sloan Digital Sky Survey Spectroscopic Lens Search. I. Discovery of Intermediate-Redshift Star-Forming Galaxies Behind Foreground Luminous Red Galaxies
• Gamma-Ray Bursts versus Quasars: Lyman-alpha Signatures of Reionization versus Cosmological Infall
• Effect of the intergalactic environment on the observability of Ly-alpha emitters during reionization
• Lyman-alpha Damping Wing Constraints on Inhomogeneous Reionization
• Detection of a huge explosion in the early Universe
• 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
• Incidence Rate of GRB-host-DLAs at High Redshift

Cited by in corpus (22)

• The Afterglows of Swift-era Gamma-Ray Bursts. I. Comparing pre-Swift and Swift era Long/Soft (Type II) GRB Optical Afterglows
• Full-Data Results of Hubble Frontier Fields: UV Luminosity Functions at $z\sim 6-10$ and a Consistent Picture of Cosmic Reionization
• Star formation in the early universe: beyond the tip of the iceberg
• GRB 100219A with X-shooter - abundances in a galaxy at z = 4.7
• GRB 130606A as a Probe of the Intergalactic Medium and the Interstellar Medium in a Star-forming Galaxy in the First Gyr After the Big Bang
• The fraction of ionizing radiation from massive stars that escapes to the intergalactic medium
• Unveiling the Secrets of Metallicity and Massive Star Formation Using DLAs along Gamma-ray Bursts
• Probing Intergalactic Neutral Hydrogen by the Lyman Alpha Red Damping Wing of Gamma-Ray Burst 130606A Afterglow Spectrum at z = 5.913
• The dark GRB080207 in an extremely red host and the implications for GRBs in highly obscured environments
• VLT/X-shooter spectroscopy of the afterglow of the Swift GRB 130606A: Chemical abundances and reionisation at $z\sim 6$
• The galaxies that reionized the Universe
• The high-redshift gamma-ray burst GRB140515A
• High Precision Analyses of Lyman alpha Damping Wing of Gamma-Ray Bursts in the Reionization Era: On the Controversial Results from GRB 130606A at z = 5.91
• Dust reddening and extinction curves towards gamma-ray bursts at z > 4
• Exploring dust extinction at the edge of reionization
• Quasars as probes of cosmological reionization
• Is GRB 050904 at z=6.3 absorbed by dust?
• A blast from the infant Universe: the very high-z GRB 210905A
• Gamma Ray Bursts as Probes of the Distant Universe
• Fires in the deep: The luminosity distribution of early-time gamma-ray-burst afterglows in light of the Gamow Explorer sensitivity requirements
• Observations of Ly$\alpha$ Emitters at High Redshift
• Spectrophotometric Redshifts. A New Approach to the Reduction of Noisy Spectra and its Application to GRB090423