# Supernova Neutrinos

Another astronomical source of neutrinos is the core-collapse supernova explosion. Massive stars with masses larger than 6−8 solar masses are very bright. However, violent delights have violent ends. When the core of a massive star runs out of nuclear fuel, it collapses under its own gravity. During the collapse, the inner core is compressed to almost the nuclear density, which has a stiff equation of state. The materials falling onto the highly compressed inner core are bounced outward which generates a shock wave and may lead to an explosion. However, supernova simulations to date show that the shock wave itself is not always energetic enough to produce the explosion 1. In most cases, it stalls and becomes a standing accretion shock 2. To revive the shock, more energy has to be deposited behind the it. A possible solution is to introduce reheating of the shock by neutrinos 1. In fact, 99% percent of the energy released in a core-collapse supernova is carried away by neutrinos. In order to implement the neutrino-driven mechanism in computer simulations of supernovae, the flux and flavor content of the neutrinos have to be known everywhere behind the shock. Thus neutrino oscillations in dense matter become a key to the supernova explosion problem.

The average energy of the neutrinos $\langle E \rangle$ emitted during a supernova explosion is of the order of 10MeV 3, and the neutrino luminosity at the early epoch of the explosion is approximately $10^{52}\mathrm{ergs\cdot s^{-1}}$ 4. Therefore, the number density of the neutrinos at the radius $R$ is $$\begin{equation*} n \sim 10^{18} \mathrm{cm^{-3}} \left(\frac{100\mathrm{km}}{R}\right)^2 \left(\frac{10\mathrm{MeV}}{\langle E \rangle}\right). \end{equation*}$$ It turns out that the ambient dense neutrino medium has a significant impact on neutrino oscillations, which has been intensely investigated in the last decade 5.

Observation-wise, the neutrino signals from a galactic supernova can reveal a great amount of information about the physical conditions inside the supernova. In fact, the detection of supernova neutrinos is on the task list of the Deep Underground Neutrino Experiment (DUNE) and many other experiments 6.

1. Hans-Thomas Janka, "Physics of core-collapse supernovae in three dimensions: A sneak preview", Annual Review of Nuclear and Particle Science 66, 341-375 (2016) . ↩︎

2. Hans A Bethe, "Revival of a stalled supernova shock by neutrino heating", The Astrophysical Journal 295, 14 (1985) . ↩︎

3. Hans-Thomas Janka, "Neutrino emission from supernovae", , 1575-1604 (2017) . ↩︎

4. Ondřej Pejcha, "The physics of the neutrino mechanism of core-collapse supernovae", The Astrophysical Journal 746, 106 (2012) . ↩︎

5. Huaiyu Duan, "Collective neutrino oscillations", Annual Review of Nuclear and Particle Science 60, 569-594 (2010) . ↩︎

6. E. Kemp, "The Deep Underground Neutrino Experiment: The precision era of neutrino physics", Astronomische Nachrichten 338, 993-999 (2017) . ↩︎

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