An Explanation for the Different X-ray to Optical Column Densities in the Environments of Gamma Ray Bursts: A Progenitor Embedded in a Dense Medium

TL;DR

This study explains the high X-ray to optical column density ratios in GRB afterglows by proposing a dense, pre-ionized environment around the progenitors, consistent with star-forming regions.

Contribution

It introduces time-evolving photoionization models showing that dense, pre-ionized clouds around GRB progenitors explain observed column density ratios, unlike homogeneous models.

Findings

Dense, pre-ionized clouds account for high X-ray to optical ratios.

Homogeneous models predict similar X-ray and optical columns, inconsistent with observations.

Cloud properties align with giant molecular clouds in star-forming regions.

Abstract

We study the > 10 ratios in the X-ray to optical column densities inferred from afterglow spectra of Gamma Ray Bursts due to gas surrounding their progenitors. We present time-evolving photoionization calculations for these afterglows and explore different conditions for their environment. We find that homogenous models of the environment (constant density) predict X-ray columns similar to those found in the optical spectra, with the bulk of the opacity being produced by neutral material at large distances from the burst. This result is independent of gas density or metallicity. Only models assuming a progenitor immersed in a dense (10^(2-4) cm-3) cloud of gas (with radius ~10 pc), with a strong, declining gradient of density for the surrounding interstellar medium are able to account for the large X-ray to optical column density ratios. However, to avoid an unphysical correlation between the size of this cloud, and the size of the ionization front produced by the GRB, the models also require that the circumburst medium is already ionized prior to the burst. The inferred cloud masses are <10^6 M_solar, even if low metallicities in the medium are assumed (Z~0.1 Z_solar). These cloud properties are consistent with those found in giant molecular clouds and our results support a scenario in which the progenitors reside within intense star formation regions of galaxies. Finally, we show that modeling over large samples of GRB afterglows may offer strong constraints on the range of properties in these clouds, and the host galaxy ISM.