Soft X‐Ray Scattering and Halos from Dust

Small angle scatterings of X-rays by interstellar dust particles create halosaround x-ray sources. By observing these halos with sufficient angularresolution it is possible to derive information about both the spatialdistribution of the dust along the line of sight to the source and some of thephysical properties of the scattering dust particles. Previous analyses ofhalos have been used to infer that the scattering dust particles must befluffy. These fluffy dust particles have also been suggested as a possiblesolution to the interstellar carbon "crisis." This crisis results from thediscrepancy between the available ISM carbon abundance and the amount requiredto be in dust to account for the average ISM extinction. Dwek (1997), however,pointed out that fluffy dust particles may not solve the carbon crisis. Inlight of this, we re-examine the evidence for fluffy dust in the ISM from x-rayscattering data. We find that previous studies overestimated the intensity of the halos byusing the RG approximation, which fails for energies below 1 keV. Using theexact Mie solution we find that below 1 keV the scattering becomes much lessefficient. Applying the Mie calculations to the x-ray halo observed around NovaCygni 1992, we conclude that the data can be explained by scattering fromregular dust, and does not require interstellar dust particles to be fluffy.Comment: latex, uses aaspp4.sty, 21 pages + 10 figures. Submitted to the Astrophysical Journa