What causes neutrino thermal radiation?

Question

Often in astrophysics, I see neutrino particles being considered as photons, in the sense that hot material emits neutrino thermal radiation. There are a lot of similarities of neutrino and photon radiation, such as the Fermi-Dirac distribution, and so on.

For the case of photons, I have some picture in mind of black body radiation being the results of vibrations of the hot material.

Is there some similar picture for neutrino emission?

Note: I do know of neutrino-emitting processes, e.g. electron capture or beta decay. My question is not about these, but rather about the emission of neutrino particles due to high temperature.

Note2: I did not differentiate in my question between the emission of different flavor neutrinos, but I am interested in that to, if an answer could direct this as well.

Answer 1

There are a number of processes that emit neutrinos - neutrino bremsstrahlung, electron capture, neutronisation, beta decay.

If these processes are in thermal equilibrium, then they are often referred to as thermal neutrinos, and the typically emitted neutrinos have energies $\sim kT$.

High temperatures and densities are normally required for these processes. For example, in the core of a supernova, the high electron density pushes the Fermi energy of the electrons high enough to cause neutronisation and electron capture reactions. The particles involved are within $\sim kT$ of the Fermi surface and thus so are the emitted neutrinos. The high density can trap the neutrinos, leading to a brief equilibrium and thermalization of the neutrino spectrum. Most processes can emit a spectrum of neutrino energies so there is neutrino opacity at most energies too. That much is similar to blackbody radiation, but of course the neutrinos have a Fermi-Dirac distribution rather than a Bose-Einstein distribution.

Answer 2

Here is one way that neutrinos can become "thermalized". This is a complex topic, and my description is a simplified version of reality:

In the core of a star that is collapsing in the early stages of a core-collapse supernova, a gigantic burst of neutrinos gets created when the tremendous pressures "down there" squeeze electrons into nuclei where they encounter protons and turn them into neutrons. Other nuclear reactions that make neutrinos start to kick in as the core collapse proceeds and a "neutrino flash" is created. This neutrino burst is considered a key property of core collapse and its observation in Supernova 1987A was a verification of our basic models of type II supernovas in general.

On their way out of the collapsing stellar core, the neutrinos would ordinarily stream straight out of it because ordinary matter is almost completely transparent to them. However, in some core collapses of supermassive stars the density of the infalling matter becomes so great that for a brief moment, even neutrinos can't freely escape, and some of them get trapped inside the core along with all the other superhot protons, neutrons and electrons.

During that moment, the neutrinos have a brief opportunity to exchange some thermal energy with with the superhot, superdense matter and with one another, a process which heats up the matter- and when the density falls a little later, the neutrinos are freed and fly off into space with an energy distribution that resembles a blackbody spectrum.

In astrophysics circles, that reheating event in which the neutrinos exchange energy with matter is called "the pause that refreshes".

I invite the experts here to weigh in with details as they see fit.