The dust scattering halo of Cygnus X-3

Abstract

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 ∝ a −p ) and fit for p, the grain radius cut-off a max , and dust mass column. We find that a p ≈ 3.5 dust grain size distribution with a max ≈ 0.2 µm 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 τ sca ∼ 2 E −2 keV . 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.

The Chandra Dust-scattering Halo of Galactic Center Transient Swift J174540.7–290015

“…We centered an annular region file on the mean pixel coordinates from the events within a 10″ radius circle of Swift J174540.7-290015. The annular bins are 1pixel wide in the inner portion and log-spaced from 6-200 pixels, to match the PSF templates in Corrales & Paerels (2015). Before extracting the profiles, we removed readout streaks, other bright objects, and all point sources in the field ( Figure 1, described in more detail below).…”

“…The high- Figure 2. 1-6 keV intensity profiles for Swift J174540.7-290015 (thick black), the respective background profile from ObsID 3392 (thick gray), and the background plus dust-free PSF (thin gray), constructed using the template method of Corrales & Paerels (2015). The contributions from the charged particle background are shown for ACIS-I (black dashed line) and ACIS-S (black dotted line ).…”

“…) and exhibits the least amount of pile-up (∼5%) of the three PSF template candidates in Corrales & Paerels (2015). Figure 2 shows the radial intensity profiles for Swift J174540.7-290015 in each observation (thick black) alongside the respective PSF reconstruction (thin gray) and background estimation from ObsID 3392 (thick gray).…”

“…where α is the observed angular distance between the point source and a scattered photon, dN/da describes the dust grain size (a) distribution, E is the photon energy, and x≡d/D describes the position of the dust grains along the sight-line, where D is the distance to the point source and d is the distance to the scatterer (e.g., Mauche & Gorenstein 1986;Smith & Dwek 1998;Corrales & Paerels 2015). Figure 4 (bottom) shows the normalized intensity profile (I F h a ) for each observation.…”

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

“…However, for a given cloud, we can expect the photo-electric absorption column to be proportional to the dust-scattering column (e.g., Corrales & Paerels 2015).…”