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We present a combined analysis of the Chandra and Swift observations of the2015 X-ray echo of V404 Cygni. Using stacking analysis, we identify eightseparate rings in the echo. We reconstruct the soft X-ray lightcurve of theJune 2015 outburst using the high-resolution Chandra images andcross-correlations of the radial intensity profiles, indicating that about 70%of the outburst fluence occurred during the bright flare at the end of theoutburst on MJD 57199.8.By deconvolving the intensity profiles with thereconstructed outburst lightcurve, we show that the rings correspond to eightseparate dust concentrations with precise distance determinations. We furthershow that the column density of the clouds varies significantly across thefield of view, with the centroid of most of the clouds shifted toward theGalactic plane, relative to the position of V404 Cyg, invalidating theassumption of uniform cloud column typically made in attempts to constrain dustproperties from light echoes. We present a new XSPEC spectral dust scatteringmodel that calculates the differential dust scattering cross section for arange of commonly used dust distributions and compositions and use it tojointly fit the entire set of Swift echo data. We find that a standardMathis-Rumpl-Nordsieck model provides an adequate fit to the ensemble of echodata. The fit is improved by allowing steeper dust distributions, and modelswith simple silicate and graphite grains are preferred over models with morecomplex composition.

A Joint Chandra and Swift View of the 2015 X-Ray Dust Scattering Echo of V404 Cygni