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I've read (in Hagai Netzer's book The Physics and Evolution of Active Galactic Nuclei) that "Comptonization in the [black hole's accretion] disk atmosphere can increase the energy of some photons..." [Section 4.2.1, Comptonization and the disk corona].

I thought Compton scattering could only decrease a photon's energy. What am I missing?

This question seems related, but it doesn't have an answer and the comments don't really shed much light on the situation.

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    $\begingroup$ Energy is obviously a quantity that depends on the frame, so whether the energy increases or decreases depends on which observer you ask. $\endgroup$ – user178876 May 13 at 1:22
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    $\begingroup$ en.wikipedia.org/wiki/… $\endgroup$ – Virgo May 13 at 1:25
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You are right, normally, when we look at coherent Thomson scattering in the accretion disk, the energy of the photons should decrease. In this case the resulting local thermal spectrum of some annulus in the disc is generally harder then a black body with the same effective temperature, due to incomplete thermalization at the scattering photosphere.

But incoherent Compton scattering in the disk surface layers by thermal electrons can reduce this spectral hardening by increasing the energy exchange between the photons and the plasma, thus raising the energy levels of the photons.

Please see here:

https://arxiv.org/pdf/1604.00070.pdf

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  • $\begingroup$ By " the ... thermal spectrum of some annulus in the disc is generally harder ...", do you mean that the thermal spectrum is of higher energy? Is that what harder means in the context of a spectrum? $\endgroup$ – Jim421616 May 13 at 2:03
  • $\begingroup$ @Jim421616 yes you are correct. $\endgroup$ – Árpád Szendrei May 13 at 2:04

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