Gamma-ray burst afterglows as probes of galactic and intergalactic dust

The amount and properties of high-redshift galactic and intergalactic (IG) dust are largely unknown but could be investigated using multiwavelength ph otometry of high-z objects that have a known intrinsic spectrum. Observatio ns of gamma -ray burst (GRB) afterglows appear to support the theoretical m odel of an adiabatic blast wave expanding into an external medium. In this model the synchrotron peak flux is independent of frequency, providing a fl at spectrum when observed over time, and therefore allowing straightforward measurement of the relative attenuation of afterglow flux in widely separa ted bands. Applying this method to dust extinction, we show that for a samp le of afterglows that have been corrected by galactic extinction comparison between the number counts of peak fluxes in X-ray versus optical can provi de constraints on an intergalactic component of dust. A similar technique c an probe the redshift dependence of extinction in GRB-forming regions witho ut requiring an assumed relation between extinction and reddening by the du st. Probing systematic changes in extinction with redshift, particularly in IG and/or nonreddening dust, is crucial to a proper interpretation of the Type Ia supernova Hubble diagram and similar observations and useful for un derstanding GRB progenitor environments.