Probing the Spatial Distribution of the Interstellar Dust Medium by High Angular Resolution X-ray Halos of Point Sources

We studied the X-ray dust scattering halos around 17 bright X-ray point sources using Chandra data. All sources were observed with the Advanced CCD Imaging Spectrometer and High-Energy Transmission Grating in Continuous Clocking Mode (CC-mode) or Timed Exposure Mode (TE-mode). We use an iterative method to resolve the halos at small scattering angles from the zeroth order data in CC-mode or the first order data in TE-mode which is not or less piled-up. Using the interstellar grain models of Weingartner & Draine (2001, WD01) and Mathis, Rumpl & Nordsieck (1977, MRN ) to fit the halo profiles, we get the hydrogen column densities and the spatial distributions of the scattering dust grains along the line of sights (LOS) to these sources. We find that the scattering dust density very close to these sources is much higher than the normal interstellar medium. For X-ray pulsars GX 301-2 and Vela X-1 with companions of strong stellar winds, the X-ray absorption column densities are much higher than the derived scattering column densities, because of the dense media around the X-ray sources produce extremely small angle scatterings which are indistinguishable from the point sources even with Chandra’s angular resolution.