Using the X-ray Dust Scattering Halo of Cygnus X-1 to determine distance and dust distributions

We present a detailed study of the X-ray dust scattering halo of the black hole candidate cygx1 based on two chandra HETGS observations. Using 18 different dust models, including one modified by us (dubbed XLNW), we probe the interstellar medium between us and this source. A consistent description of the cloud properties along the line of sight that describes at the same time the halo radial profile, the halo lightcurves, and the column density from source spectroscopy is best achieved with a small subset of these models. Combining the studies of the halo radial profile and the halo lightcurves, we favor a geometric distance to cygx1 of $d=1.81pm{0.09}$,kpc. Our study also shows that there is a dense cloud, which contributes $sim$50% of the dust grains along the line of sight to cygx1, located at $sim1.6$ kpc from us. The remainder of the dust along the line of sight is close to the black hole binary.

The accretion disk corona and disk atmosphere of 4U 1624-490 as viewed by the Chandra-HETGS

We present a detailed spectral study (photoionization modelling and variability) of the Big Dipper 4U 1624-490 based on a chandra-High Energy Transmission Gratings Spectrometer (HETGS) observation over the $sim76$ ks binary orbit of 4U 1624-490. While the continuum spectrum can be modeled using a blackbody plus power-law, a slightly better fit is obtained using a single $Gamma=2.25$ power-law partially (71%) covered by a local absorber of column density $N_{rm H, Local}=8.1_{-0.6}^{+0.7}times 10^{22} rm cm^{-2}$. The data show a possible quasi-sinusoidal modulation with period $43_{-9}^{+13}$ ks that might be due to changes in local obscuration. Photoionization modeling with the {sc xstar} code and variability studies of the observed strong ion{Fe}{25} and ion{Fe}{26} absorption lines point to a two-temperature plasma for their origin: a highly ionized component of ionization parameter $xi_{rm hot} approx 10^{4.3} {rm ergs cm s^{-1}}$ ($Tsim 3.0times 10^{6}$ K) associated with an extended accretion disk corona of radius $R sim3times10^{10}$ cm, and a less ionized more variable component of $xi approx 10^{3.4} {rm ergs cm s^{-1}}$ ($Tsim 1.0times 10^{6}$ K) and $rm xi approx 10^{3.1} ergs cm s^{-1}$ ($Tsim 0.9times 10^{6}$ K) coincident with the accretion disk rim. We use this, with the observed ion{Fe}{25} and ion{Fe}{26} absorption line variations (in wavelength, strength, and width) to construct a viewing geometry that is mapped to changes in plasma conditions over the 4U 1624-490 orbital period.