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@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. $\endgroup$
    – ProfRob
    Mar 18, 2019 at 7:16
  • $\begingroup$ @ProfRob 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
    May 23, 2022 at 18:28

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Most neutron stars? I would think not. The cooling time for a neutron star is initially seconds due to neutrino emission and thermal conduction is very effective, so the bulk of the neutron star will be isothermal. There will be a thin non-degenerate layer on top of the crust with a big temperature drop to the "photosphere". By the time a neutron star is a few years old, its surface will be $<10^7$ K and maybe $10^6$ K after 1000 years. These temperatures are far too low to produce significant thermal radiation above 10 keV. Of course, the interior could be a factor of $\sim 100$ hotter and will produce hard X-rays, but these will not escape because the layers above will be thick to Compton scattering and inverse bremsstrahlung, which involve both nuclei and electrons.

There will however be a soft X-ray photosphere in young neutron stars. Opacity will be provided by the photoelectric effect, Compton/Thomson scattering and inverse bremsstrahlung.

The "photosphere" becomes just about detectable at (soft) X-ray energies during accretion bursts from companion stars (e.g. Cottam et al. 2008). But even here, the thermal spectrum is only a few keV in temperature. This is still far too cool to produce gamma rays, but there are observations that show (perhaps) absorption features in the soft X-ray spectrum in much the same way as we see Fraunhofer lines in the visible solar photosphere. The burst spectrum is contaminated though by elements in the accreted material.

The X-ray photosphere of a neutron star The X-ray photospheric spectrum during an X-ray burst (from Cottam et al. 2008).

Hard X-rays and gamma rays from neutron stars will not be photospheric, they will arise from non-thermal processes associated with charged particle acceleration in the strong electromagnetic fields that likely thread the surface.

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