The Dust Scattering Halo of Cygnus X-3

Dust grains scatter X-ray light through small angles, producing a diffuse halo image around bright X-ray point sources situated behind a large amount of interstellar material. We present analytic solutions to the integral for the dust scattering intensity, which allow for a Bayesian analysis of the scattering halo around Cygnus X-3. Fitting the optically thin 4-6 keV halo surface brightness profile yields the dust grain size and spatial distribution. We assume a power law distribution of grain sizes ($n \propto a^{-p}$) and fit for $p$, the grain radius cut-off amax, and dust mass column. We find that a $p \approx 3.5$ dust grain size distribution with amax $\approx 0.2$ um fits the halo profile relatively well, whether the dust is distributed uniformly along the line of sight or in clumps. We find that a model consisting of two dust screens, representative of foreground spiral arms, requires the foreground Perseus arm to contain 80\% of the total dust mass. The remaining 20\% of the dust, which may be associated with the outer spiral arm of the Milky Way, is located within 1 kpc of Cyg X-3. Regardless of which model was used, we found $\tau_{\rm sca} \sim 2 \ E_{\rm keV}^{-2}$. We examine the energy resolved halos of Cyg X-3 from 1 - 6 keV and find that there is a sharp drop in scattering halo intensity when $E < 2-3$ keV, which cannot be explained with multiple scattering effects. We hypothesize that this may be caused by large dust grains or material with unique dielectric properties, causing the scattering cross-section to depart from the Rayleigh-Gans approximation that is used most often in X-ray scattering studies. The foreground Cyg OB2 association, which contains several evolved stars with large extinction values, is a likely culprit for grains of unique size or composition.