Neutrino Emission from Supernovae (1702.08713v1)

Abstract: Supernovae are the most powerful cosmic sources of MeV neutrinos. These elementary particles play a crucial role when the evolution of a massive star is terminated by the collapse of its core to a neutron star or a black hole and the star explodes as supernova. The release of electron neutrinos, which are abundantly produced by electron captures, accelerates the catastrophic infall and causes a gradual neutronization of the stellar plasma by converting protons to neutrons as dominant constituents of neutron star matter. The emission of neutrinos and antineutrinos of all flavors carries away the gravitational binding energy of the compact remnant and drives its evolution from the hot initial to the cold final state. The absorption of electron neutrinos and antineutrinos in the surroundings of the newly formed neutron star can power the supernova explosion and determines the conditions in the innermost supernova ejecta, making them an interesting site for the nucleosynthesis of iron-group elements and trans-iron nuclei. In this Chapter the basic neutrino physics in supernova cores and nascent neutron stars will be discussed. This includes the most relevant neutrino production, absorption, and scattering processes, elementary aspects of neutrino transport in dense environments, the characteristic neutrino emission phases with their typical signal features, and the perspectives connected to a measurement of the neutrino signal from a future galactic supernova.

Neutrino Emission from Supernovae

The document authored by Hans-Thomas Janka presents a comprehensive paper of the role of neutrinos during the collapse of massive stars, resulting in supernovae and subsequent formation of either neutron stars or black holes. The research elucidates the intricate processes through which neutrinos, predominantly electron neutrinos, dynamically impact the collapse, explosion, and nucleosynthesis in supernovae.

Supernovae serve as extraordinarily powerful sources of MeV neutrinos, pivotal in carrying away the gravitational binding energy post-core collapse. This paper explores how the emission of neutrinos and antineutrinos from supernova cores transports energy, facilitating the transition of the evolving neutron star from a hot, dense state to a cooler configuration. The author's foundational discussion includes neutrino production, absorption, and scattering processes occurring within these dense stellar environments, as well as the characteristics of neutrino emission phases.

Throughout the stages of core-collapse, the neutrino production mechanisms are dominated by electron capture and pair production processes, with varying neutrino flavors playing essential roles at different phases. During core collapse, electron neutrinos are predominantly produced, accelerating the infall and initiating neutronization. This rise in neutrino density leads to significant neutrino trapping within the core as densities climb to around $10^{11}$ g cm$^{-3}$.

Upon reaching nuclear densities, the core undergoes a bounce, forming a shockwave temporarily halting the collapse. As the shock propagates outward, crossing the trapping density threshold, it liberates a burst of $\nu_e$, characterizing the shock-breakout neutrino burst. This burst is significant, as it emits a substantial fraction of total neutrino energy within milliseconds.

Subsequent stages involve neutrino-driven convection and hydrodynamic instabilities, such as the standing accretion-shock instability (SASI), which contribute significantly to the revival of the stalled shock in what is hypothesized as the neutrino-driven mechanism for supernova explosions.

One crucial element underscored is the flavor-dependent decoupling and interaction of neutrinos with dense matter. The neutrino spectra exhibit evidence of pinching, deviating from perfect thermal distributions due to energy-dependent decoupling radii and interaction cross-sections. This aspect is pivotal for understanding the impact of neutrinos on nucleosynthesis processes occurring in the ejected stellar material.

The implications of these findings are profound, offering insights not only into stellar evolution and collapse mechanisms but also providing observable phenomena - such as those from SN 1987A - valuable for refining theoretical models. The paper speculates on the extent to which neutrino signals from future galactic supernovae could enhance our understanding of supernova physics, particularly through improved neutrino transport simulations and consideration of flavor oscillations.

Overall, Janka's work encapsulates a detailed and critical investigation into the essential physics of neutrino emissions during supernova events. This research adds depth to our comprehension of core-collapse supernova dynamics and establishes a framework for analyzing future supernova neutrino signals which may, in turn, yield information on fundamental astrophysical processes and the properties of newly formed neutron stars. Future advancements in supernova modeling are anticipated to tackle the multidimensional complexities deepening insights into the neutrino-driven explosion mechanism.