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

X-ray bright quasars might be used to trace dust in the circumgalactic and intergalactic medium through the phenomenon of X-ray scattering, which is observed around Galactic objects whose light passes through a sufficient column of interstellar gas and dust. Of particular interest is the abundance of grey dust larger than 0.1 um, which is difficult to detect at other wavelengths. To calculate X-ray scattering from large grains, one must abandon the traditional Rayleigh-Gans approximation. The Mie solution for the X-ray scattering optical depth of the Universe is ~1%. This presents a great difficulty for distinguishing dust scattered photons from the point source image of Chandra, which is currently unsurpassed in imaging resolution. The variable nature of AGN offers a solution to this problem, as scattered light takes a longer path and thus experiences a time delay with respect to non-scattered light. If an AGN dims significantly (> 3 dex) due to a major feedback event, the Chandra point source image will be suppressed relative to the scattering halo, and an X-ray echo or ghost halo may become visible. I estimate the total number of scattering 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 capable of dimming an AGN quickly.