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 and Draine and Mathis and coworkers to fit the halo profiles, we get the hydrogen column densities and the spatial distributions of the scattering dust grains along the lines of sight (LOSs) 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 the dense media around the X-ray sources produce extremely small angle scatterings that are indistinguishable from point sources, even with Chandra's angular resolution. For LMCX-1, most of the scattering and absorption occur in the Large Magellanic Cloud, rather than in the Milky Way. From the obtained X-ray spectra, the cold gas absorption and thus the equivalent hydrogen column are determined. We have obtained the linear correlation between the N-H derived from spectral fits and that derived from the grain models of Weingartner and Draine and Mathis and coworkers (except for GX 301-2 and Vela X-1): N-H; WD01 = (0.720 +/- 0.009)N-H,N-abs + (0.051 +/- 0.013) and N-N,N-MRN = (1.156 +/- 0.016)N-H,N-abs + (0.062 +/- 0.024) in units of 10(22) cm(-2). High angular resolution X-ray dust scattering halos offer an excellent potential for probing the spatial distributions of the interstellar dust medium.