The Molecular Hydrogen Deficit in Gamma-Ray Burst Afterglows

Abstract

Recent analysis of five gamma-ray burst (GRB) afterglow spectra reveal the absence of molecular hydrogen absorption lines, a surprising result in light of their large neutral hydrogen column densities and the detection of H${}_2$ in similar, more local star-forming regions like 30 Doradus in the Large Magellanic Cloud (LMC). Observational evidence further indicates that the bulk of the neutral hydrogen column in these sight lines lies 100 pc beyond the progenitor and that H${}_2$ was absent prior to the burst, suggesting that direct flux from the star, FUV background fields, or both suppressed its formation. We present one-dimensional radiation hydrodynamical models of GRB host galaxy environments, including self-consistent radiative transfer of both ionizing and Lyman-Werner photons, nine-species primordial chemistry with dust formation of H${}_2$, and dust extinction of UV photons. We find that a single GRB progenitor is sufficient to ionize neutral hydrogen to distances of 50 - 100 pc but that a galactic Lyman-Werner background is required to dissociate the molecular hydrogen in the ambient ISM. Intensities of 0.1 - 100 times the Galactic mean are necessary to destroy H${}_2$ in the cloud, depending on its density and metallicity. The minimum radii at which neutral hydrogen will be found in afterglow spectra is insensitive to the mass of the progenitor or the initial mass function (IMF) of its cluster, if present.

12 pages, 7 figures, accepted for ApJ

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• Molecular hydrogen in high-redshift Damped Lyman-alpha systems: The VLT/UVES database
• Very different X-ray to optical column density ratios in gamma-ray burst afterglows: ionisation in GRB environments
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Cited by in corpus (4)

• The First Positive Detection of Molecular Gas in a GRB Host Galaxy
• High-Redshift Starbursting Dwarf Galaxies Revealed by GRB Afterglows
• A Survey for NV Absorption at z~z_GRB in GRB Afterglow Spectra: Clues to Gas Near the Progenitor Star
• Constraining the star formation histories of GRB Host Galaxies from their observed abundance patterns