Determining the Dust Extinction of Gamma-ray Burst Host Galaxies: A Direct Method Based on Optical and X-ray Photometry

TL;DR

This paper introduces a direct optical-X-ray photometry method to measure dust extinction in GRB host galaxies, revealing large, gray extinction curves and skewed dust grain size distributions, with implications for understanding GRB environments.

Contribution

It presents a novel, straightforward method to determine dust extinction in GRB host galaxies by comparing optical and X-ray spectra, and models the extinction curves to infer dust properties.

Findings

Derived large, gray extinction curves unlike local galaxies.

Estimated high visual extinction values (A_V ~ 1.6-3.4 mag).

Determined dust-to-gas ratios in GRB host galaxies for the first time.

Abstract

The dust extinction of gamma-ray bursts (GRBs) host galaxies, containing important clues to the nature of GRB progenitors and crucial for dereddening, is still poorly known. Here we propose a straightforward method to determine the extinction of GRB host galaxies by comparing the observed optical spectra to the intrinsic ones extrapolated from the X-ray spectra. The rationale for this method is from the standard fireball model: if the optical flux decay index equals to that of the X-ray flux, then there is no break frequency between the optical and X-ray bands, therefore we can derive the intrinsic optical flux from the X-ray spectra. We apply this method to three GRBs of which the optical and X-ray fluxes have the same decay indices and another one with inferred cooling break frequency, and obtain the rest-frame extinction curves of their host galaxies. The derived extinction curves are gray and do not resemble any extinction curves of local galaxies (e.g. the Milk Way, the Small/Large Magellanic Clouds, or nearby starburst galaxies). The amount of extinction is rather large (with visual extinction $A_V$ $\sim$ 1.6--3.4$\magni$). We model the derived extinction curves in terms of the silicate-graphite interstellar grain model. As expected from the ``gray'' nature of the derived extinction curve, the dust size distribution is skewed to large grains. We determine, for the first time, the local dust-to-gas ratios of GRB host galaxies using the model-derived dust parameters and the hydrogen column densities determined from X-ray absorptions.