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.
