X-RAY SCATTERING ECHOES AND GHOST HALOS FROM THE INTERGALACTIC MEDIUM: RELATION TO THE NATURE OF AGN VARIABILITY
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Corrales-2015-X-RAY SCATTERING ECH.pdf
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Author(s)
Corrales, Lia
Date Issued
May 2015
Journal
The Astrophysical Journal
Publisher
IOP Publishing
Citation
Corrales, Lia. “X-RAY SCATTERING ECHOES AND GHOST HALOS FROM THE INTERGALACTIC MEDIUM: RELATION TO THE NATURE OF AGN VARIABILITY.” The Astrophysical Journal 805, no. 1 (May 15, 2015): 23. © 2015 The American Astronomical Society
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Final published version
Abstract
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 gray dust larger than 0.1 µm, 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 AGNs 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 ([> over ~]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[subscript ech] ~ 10[superscript 3](ν[subscript fb]/yr[superscript -1]), where ν[subscript fb] is the characteristic frequency of feedback events capable of dimming an AGN quickly.
MIT Department
MIT Kavli Institute for Astrophysics and Space Research
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DOI of Published Version
https://doi.org/10.1088/0004-637x/805/1/23
