Deriving gamma-ray burster distances from soft X-ray measurements

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Perhaps the most startling conclusion that can be inferred from observations by the Burst and Transient Source Experiment is that the long-standing question of burster origins may not be answered by gamma-ray measurements alone. The data show that the measured spatial distribution of burst sources is both isotropic and sampled to its characteristic distance - meaning that it is very unlikely that they are related to a Galactic population of neutron stars. When coupled with the lack of an identifiable counterpart at any wavelength or a convincing correlation with any astronomical population, the data have led to a confused proliferation of models. Whereas models tend to be vague on much of the burst details, they can at least be grouped according to a distance scale. We contend that the burster origin problem can best be solved by establishing that scale. Although several techniques have been proposed, we show that this can be easily carried out at soft X-ray wavelengths by measuring the effective column densities of a representative sample of burst spectra. Even if the result does not lead to a unique association with a particular class of object, it will eliminate whole classes of models by providing the first quantitative description of the three-dimensional distribution of burst sources in the universe. In this paper, we demonstrate the feasibility of this technique, both scientifically and technically, and explore actual and perceived problems.

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Astrophysical Journal

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