@RobJeffries' answer to Is there evidence of super-heavy elements in the x-ray and gamma-ray spectrums of neutron stars? includes the sentence:

However, the only things that contribute to a neutron star's observable spectrum are materials within a few cm of the neutron star surface.

and that reminds me a little bit of the Sun's photosphere. For our Sun, visible photons produced below the photosphere don't make it out, at least not without their spectrum being altered by scattering.

I think a similar thing can be said about the X-rays and gamma rays from inside a neutron star.

Question: Do neutron stars have something like a gamma-ray photosphere? Are gamma rays from below it limited more by the nuclei or electrons?

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    $\begingroup$ It is a photosphere. What provides the opacity? Yes, nuclei and electrons. Nuclei provide absorption at discrete frequencies, that are doubtless severely broadened by pressure and magnetic fields. Electrons provide Compton scattering. Photoelectric absorption and pair production will also feature. $\endgroup$ – Rob Jeffries Mar 18 '19 at 7:16
  • $\begingroup$ @RobJeffries I'm not surprised, thanks! I know it's a complex topic, but if restricted to say tens to hundreds of keV gamma rays that could be used to identify isotopes and some representative neutron star if such a thing exists, would that be narrow enough to write a short answer? $\endgroup$ – uhoh Mar 18 '19 at 7:42

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