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.

Condensed Matter Astrophysics: A Prescription for Determining the Species-Specific Composition and Quantity of Interstellar Dust using X-rays

We present a new technique for determining the *quantity and composition* of dust in astrophysical environments using <6keV X-rays. We argue that high resolution X-ray spectra as enabled by the Chandra and XMM-Newton gratings should be considered a powerful and viable new resource for delving into a relatively unexplored regime for directly determining dust properties: composition, quantity, and distribution. We present initial cross-section measurements of astrophysically likely iron-based dust candidates taken at the Lawrence Berkeley National Laboratory Advanced Light Source synchrotron beamline, as an illustrative tool for the formulation of our methodology. Focused at the 700eV Fe LIII and LII photoelectric edges, we discuss a technique for modeling dust properties in the soft X-rays using L-edge data, to complement K-edge X-ray absorption fine structure analysis techniques discussed in Lee & Ravel 2005. This is intended to be *a techniques paper* of interest and usefulness to both condensed matter experimentalists and astrophysicists. For the experimentalists, we offer a new prescription for normalizing relatively low S/N L-edge cross section measurements. For astrophysics interests, we discuss the use of X-ray absorption spectra for determining dust composition in cold and ionized astrophysical environments, and a new method for determining *species-specific gas-to-dust ratios*. Possible astrophysical applications of interest, are offered. Prospects for improving on this work with future X-ray missions with higher throughput and spectral resolution are presented in the context of spectral resolution goals for gratings and calorimeters, for proposed and planned missions such as Astro-H and the International X-ray Observatory.

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.

A Study of the X-Ray Dust Scattering Halo of Cyg X-1 with a Cross-Correlation Method

X-ray photons scattered by the interstellar medium carry information about dust distribution, dust grain model, scattering cross section, and the distance of the source; they also take longer time than unscattered photons to reach the observer. Using a cross-correlation method, we study the light curves of the X-ray dust scattering halo of Cyg X-1, observed with the textit{Chandra X-ray Observatory}. Significant time lags are found between the light curves of the point source and its halo. This time lag increases with the angular distance from Cyg X-1, implying a dust concentration at a distance along the line of sight (LOS) of 2.0 kpc $times$ (0.876 $pm$ 0.002) from the Earth. By fitting the observed light curves of the halo at different radii with simulated light curves, we obtain a width of $mathit{Delta L}=33_{-13}^{+18}$ pc of this dust concentration. The origin of this dust concentration is still not clearly known. The advantage of our method is that we need no assumption of scattering cross section, dust grain model, or dust distribution along the LOS. Combining the derived dust distribution from the cross-correlation study with the surface brightness distribution of the halo, we conclude that the two commonly accepted models of dust grain size distribution need to be modified significantly.