Here’s an important ponderable: what does neutronium look like? Further, what does intensely hot neutronium look like?
As to the former, my guess is that it would look like a very reflective mirror. Photons would seem likely to simply bounce off. But for reasons that don’t bear going into, I was thinking about extremely hot neutronium, such as might be found in a recently created neutron star. Hot materials, such as molten iron, emit light via incandescence: as energetic electrons drop into lower energy levels, the difference in energy is spat out in the form of a photon. But what would be the mechanism for a neutron to emit a photon? Or… can a lump of neutronium even be said to actually have a temperature in the first place? High-energy neutrons are often encountered, but in the form of free-flying individuals, with energy measured as a result of velocity. But locked into a stable high-pressure, high density neutron fluid would seem to negate velocity.
So, with no electrons to go up or down in energy states, there would seem to be limited means to emit photons. If that’s correct, neutronium fresh out of the supernova factory might be at billions of degrees, but it would look just like neutronium that had somehow been cooled to cryogenic temperatures.
Additionally: if this is correct, a billion-degree lump of neutronium doesn’t seem to have a heat rejection mechanism. A billion years down the line, it’ll be just as hot as today.
So… what am I missing/
Note: unless some sort of technomagical incredibly strong force field is developed, there will probably never be a lump of neutronium of less than stellar mass. Without the neutron stars monumental gravity keeping the neutronium compressed, it’ll simply explode.