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X-ray bright quasars might be used to trace dust in the circumgalactic andintergalactic medium through the phenomenon of X-ray scattering, which isobserved around Galactic objects whose light passes through a sufficient columnof interstellar gas and dust. Of particular interest is the abundance of greydust larger than 0.1 um, which is difficult to detect at other wavelengths. Tocalculate X-ray scattering from large grains, one must abandon the traditionalRayleigh-Gans approximation. The Mie solution for the X-ray scattering opticaldepth of the Universe is ~1%. This presents a great difficulty fordistinguishing dust scattered photons from the point source image of Chandra,which is currently unsurpassed in imaging resolution. The variable nature ofAGN offers a solution to this problem, as scattered light takes a longer pathand thus experiences a time delay with respect to non-scattered light. If anAGN dims significantly (> 3 dex) due to a major feedback event, the Chandrapoint source image will be suppressed relative to the scattering halo, and anX-ray echo or ghost halo may become visible. I estimate the total number ofscattering echoes visible by Chandra over the entire sky: N_ech ~ 10^3 (nu_fb /yr^-1), where nu_fb is the characteristic frequency of feedback events capableof dimming an AGN quickly.

X-ray Scattering Echoes and Ghost Halos from the Intergalactic Medium: Relation to the nature of AGN variability