Inferring the Spatial and Energy Distribution of Gamma‐Ray Burst Sources. III. Anisotropic Models

Abstract

We use Bayesian methods to study anisotropic models for the distribution of gamma-ray burst intensities and directions reported in the Third BATSE Catalog (3B catalog) of gamma-ray bursts. We analyze data obtained using both the 64 ms and 1024 ms measuring timescales. We study both purely local models, in which burst sources ("bursters") are presumed to be distributed in extended halos about the Galaxy and M31, and mixed models consisting of a cosmological population of standard-candle bursters and a local population distributed throughout a standard Bahcall-Soneira dark matter halo with a 2 kpc core. In a companion paper we study isotropic models, including a variety of cosmological models, using the same methodology adopted here, allowing us to rigorously and quantitatively compare isotropic and anisotropic models. We find that the purely local models we have studied can account for the 3 B data as successfully as cosmological models, provided one considers halos with core sizes significantly larger than those used to model the distribution of dark matter. A preference for cosmological over local models, or vice versa, must therefore be justified by means of information other than the distribution of burst directions and intensities. We infer core sizes for the halo distribution that are smaller than one might expect based on popular semiquantitative arguments that consider the superposed dipole moments of shells centered on the Galactic center and show why such arguments can lead to unwarranted conclusions. We also find that the 3B data do not constrain the width of power-law luminosity functions for burst sources. This disagrees with the findings of previous studies; we elucidate the qualitative reasons for the lack of a constraint, and discuss why our results differ from those of earlier studies. Our analysis of mixed models finds two families of models that can successfully account for the data: models with up to 20% of observed bursts in a bright local population visible to ∼50 kpc and models with up to 50% of observed bursts in a dim local population visible only nearby (to less than a disk scale height). These models fit as well or better than purely cosmological models. They indicate that a surprisingly large local, anisotropic component could be present whose size is comparable to the sizes of hypothetical classes of bursts inferred from analyses of temporal and spectral characteristics. Finally, as in our study of isotropic models, we find substantial systematic differences between results based on 64 ms and 1024 ms data, which indicates that a thorough understanding of the distribution of burst tensities and directions is likely to require detailed analysis of temporal properties. © 1998. The American Astronomical Society. All rights reserved.