The Properties of Interstellar Dust From a Survey of X-ray Halos
Abstract
Small-angle scattering of X-rays off interstellar dust grains produce X-ray halos around bright sources along absorbed lines of sight. While many studies have examined these halos, no systematic study has compared the available halo data to the large number of dust models that are commonly used. To address this, we have obtained the largest sample yet of X-ray halos from XMM-Newton and Chandra and fitted them with 14 dust grain models. We have also compared our results with the optical extinction, AV, when it is available in the literature. Our results can be summarized as follows. (1) Comparing AV with NH values measured by X-ray spectral fitting, we find a ratio of AV/NH (1021 cm-2) = 0.48 ± 0.06, similar to what has been found by previous workers. (2) Out of 35 halos, 27 could be fit by one or more grain models, with the most successful models having maximum grain radius amax < 0.4 μm and fewer large grains than the less successful models. This suggests that the diffuse ISM does not contain a signicant presence of grains with amax > 0.5 μm. (3) Most halos were best fit assuming a single dust cloud dominated the scattering, as opposed to smoothly distributed dust along the sight line. (4) Eight sources could not be fit with the models considered here; these tended to be distant (d > 5 kpc) sight lines through the Galactic thin disk. 5) Some sight lines had halos with observed X-ray scattering optical depth τsca/AV that were significantly different than expected, which may be indicative of an inhomogeneous dust distribution across the halo extraction area on the sky.
- Publication:
-
AAS/High Energy Astrophysics Division #15
- Pub Date:
- April 2016
- Bibcode:
- 2016HEAD...1511009V