Future Constraints on the Reionization History and the Ionizing Sources from Gamma-Ray Burst Afterglows

Adam Lidz,Lluís Mas-Ribas,Tzu-Ching Chang,Guochao Sun

doi:10.3847/1538-4357/ac0af0

Abstract

Abstract We forecast the reionization history constraints, inferred from Lyα damping wing absorption features, for a future sample of ∼20 z ≥ 6 gamma-ray burst (GRB) afterglows. We describe each afterglow spectrum by a three-parameter model. First, L characterizes the size of the ionized region (the “bubble size”) around a GRB host halo. Second, 〈x H i 〉 is the volume-averaged neutral fraction outside of the ionized bubble around the GRB, which is approximated as spatially uniform. Finally, N H i denotes the column density of a local damped Lyα absorber (DLA) associated with the GRB host galaxy. The size distribution of ionized regions is extracted from a numerical simulation of reionization and evolves strongly across the epoch of reionization (EoR). The model DLA column densities follow the empirical distribution determined from current GRB afterglow spectra. We use a Fisher matrix formalism to forecast the 〈x H i (z)〉 constraints that can be obtained from follow-up spectroscopy of afterglows with S/N = 20 per R = 3000 resolution element at the continuum. We find that the neutral fraction may be determined to better than 10%–15% (1σ) accuracy from this data across multiple independent redshift bins at z ∼ 6–10, spanning much of the EoR, although the precision degrades somewhat near the end of reionization. A more futuristic survey with 80 GRB afterglows at z ≥ 6 can improve the precision here by a factor of 2 and extend measurements out to z ∼ 14. We further discuss how these constraints may be combined with estimates of the escape fraction of ionizing photons derived from the DLA column density distribution toward GRBs extracted at slightly lower redshift, z ∼ 5. This combination will help in testing whether we have an accurate census of the sources that reionized the universe.

Highlights

Gamma-ray bursts (GRBs) are, electromagnetically, the brightest explosions in the universe
GRB afterglows can be exploited as bright backlights, with follow-up spectroscopy of the afterglows at high spectral resolution and signal-to-noise ratio (S/N) providing valuable constraints on the neutral fraction of the intergalactic medium (IGM; e.g., Totani et al 2006; Hartoog et al 2015) and the chemical enrichment history of the interstellar media (ISM) in the GRB host galaxy (e.g., Lamb & Reichart 2000; Savaglio 2006)
We find that an 8 m telescope can achieve an S/N of 20 on a 10 μJy GRB afterglow in tobs = 6.1 hr of observing time at a spectral resolution of R = 3000

Summary

Introduction

Gamma-ray bursts (GRBs) are, electromagnetically, the brightest explosions in the universe. The long-duration class of GRBs is thought to be powered by the collapse of massive rotating stars (Woosley 1993); as nuclear fuel is exhausted at the end of the star’s life, the core loses pressure support and implodes to form a black hole. In this process, jets of energetic particles are produced, which plow through the remaining stellar envelope. Jets of energetic particles are produced, which plow through the remaining stellar envelope This relativistic outflow leads to highly beamed gamma-ray emission, while subsequent afterglows occur at longer wavelengths as the ejecta runs into surrounding gas in the circumburst and interstellar media (ISM). GRB afterglows can be exploited as bright backlights, with follow-up spectroscopy of the afterglows at high spectral resolution and signal-to-noise ratio (S/N) providing valuable constraints on the neutral fraction of the intergalactic medium (IGM; e.g., Totani et al 2006; Hartoog et al 2015) and the chemical enrichment history of the ISM in the GRB host galaxy (e.g., Lamb & Reichart 2000; Savaglio 2006)

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Conclusion