Probing intergalactic neutral hydrogen by the Lyman alpha red damping wing of gamma-ray burst 130606A afterglow spectrum at z = 5.913

Abstract

The unprecedentedly bright optical afterglow of GRB 130606A located by Swift at a redshift close to the reionization era (z = 5.913) provides a new opportunity to probe the ionization status of the intergalactic medium (IGM). Here we present an analysis of the red Lyα damping wing of the afterglow spectrum taken by Subaru/FOCAS during 10.4–13.2 hr after the burst. We find that the minimal model including only the baseline power-law and H i absorption in the host galaxy does not give a good fit, leaving residuals showing concave curvature in 8400–8900 Å with an amplitude of about 0.6% of the flux. Such a curvature in the short wavelength range cannot be explained either by extinction at the host with standard extinction curves, intrinsic curvature of afterglow spectra, or by the known systematic uncertainties in the observed spectrum. The red damping wing by intervening H i gas outside the host can reduce the residual by about 3 σ statistical significance. We find that a damped Lyα system is not favored as the origin of this intervening H i absorption, from the observed Lyβ and metal absorption features. Therefore absorption by diffuse IGM remains as a plausible explanation. A fit by a simple uniform IGM model requires an H i neutral fraction of fH i ∼ 0.1–0.5 depending on the distance to the GRB host, implying high fH i IGM associated with the observed dark Gunn–Peterson (GP) troughs. This gives new evidence that the reionization is not yet complete at z = 6.

The duration of reionization constrains the ionizing sources

“…An estimate of the duration of reionization can be obtained by studying the evolution of the ionized fraction. We calculate the evolution of ionized fraction and compare with existing constraints obtained from various observations: Lyα dark gap statistics (McGreer et al 2015)), the IGM damping wings in a z = 7 quasar (Mortlock et al 2011), the damping wing in a Gamma-ray burst (Totani et al 2014), galaxy clustering (McQuinn et al 2007), Lyα emitters (Ota et al 2008;Ouchi et al 2010) and the Lyα emission statistics of galaxies (Caruana et al 2012;Tilvi et al 2014;Schenker et al 2014).…”

The MUSE Hubble Ultra Deep Field Survey

“…Gunn & Peterson 1965;Fan et al 2006;McGreer et al 2015, see however, Bosman et al 2018) and in gamma-ray burst spectra (e.g. Totani et al 2006Totani et al , 2014 suggest that cosmic reionization was completed by z ≈ 6. The Thomson optical depth of the cosmic microwave background measured by Planck suggests that the midpoint redshift of reionization (i.e.…”

GRBs as Probes of the IGM

“…The afterglow was observed spectroscopically with several instruments/telescopes: from the low-resolution FOCAS camera on Subaru telescope (resolving power of R ∼ 900) to the X-Shooter instrument on VLT (R ∼ 10000). Here we present the results from the former dataset and discuss some discrepancies with other groups' conclusions Totani et al 2014;Hartoog et al 2015).…”

“…For several years, only the spectrum of GRB 050904(at z = 6.295 ) was able to provide constraints on the neutral hydrogen fraction (f HI < 0.6), based solely on analysis of the red-damping wing. Recently, a newly discovered GRB at z = 5.912, GRB 130606A, has provided a great dataset for a similar analysis Totani et al 2014;Hartoog et al 2015): the bright optical/NIR afterglow enabled not only a large follow-up effort, but absorption spectroscopy has shown a low HI column density in the host, implying a much better constraint on the actual f HI of the IGM. GRB 130606A was discovered by the Swift and KONUS-Wind (Golenetskii et al 2013) and its redshift was identified based on HI and metal absorption features at the same redshift (z = 5.912).…”

“…Regions of the spectrum cleaned of absorption features like C ii or N v, as well as strong skyline emission, were excluded by the fit and simple chi-squared minimization statistics were employed in order to find the best model. The best-fit model parameters are listed in Table 1 of Totani et al (2014), and provide a neutral hydrogen fraction of f HI = 0.086 +0.012 −0.011 . Furthermore, we were able to exclude with high-confidence a model that contains only a host DLA, while one or two components (diffuse IGM only, or IGM+intervening DLA) are equally valuable good fits ( Figure 5).…”