Measuring interstellar dust grains from the haloes of binary X-ray sources

Abstract

Overbeck1 first predicted that the images of cosmic X-ray sources might have observable haloes due to coherent forward scattering by interstellar dust grains, and it was recognized that the observations of these haloes would provide a powerful tool for analysing the size distribution and composition of the grains2–5. Observations with the Einstein observatory have provided evidence of faint haloes extending to a few arc minutes beyond the X-ray images of supernova remnants6,7 and several compact galactic sources8–11. The evidence that the haloes are actually due to interstellar grain scattering appears substantial, but it is subject to some uncertainty resulting from the dependence of the point response function (PRF) of the telescope on the source spectra (the observed haloes are brighter than the predicted PRF by factors of approximately 1.5–2). In this letter we show how observations of the time-dependence of the halo X-ray emission may be analysed to determine the location, size distribution, and composition of interstellar grains. Indeed, it should be possible to observe and analyse the halo emission during eclipse even without imaging the source, and we suggest that such haloes probably account for the soft X-rays that have been observed during eclipses or dips in several binary systems.