Neutrino Signatures from Young Neutron Stars

Abstract

After a successful core collapse supernova (CCSN) explosion, a hot dense proto-neutron star (PNS) is left as a remnant. Over a time of twenty or so seconds, this PNS emits the majority of the neutrinos that come from the CCSN, contracts, and loses most of its lepton number. This is the process by which all neutron stars in our galaxy are likely born. The emitted neutrinos were detected from SN 1987A and they will be detected in much greater numbers from any future galactic CCSN. These detections can provide a direct window into the properties of the dense matter encountered inside neutron stars, and they can affect nucleosynthesis in the material ejected during the CCSN. In this chapter, we review the basic physics of PNS cooling, including the basic equations of PNS structure and neutrino diffusion in dense matter. We then discuss how the nuclear equation of state, neutrino opacities in dense matter, and convection can shape the temporal behavior of the neutrino signal. We also discuss what was learned from the late time SN 1987A neutrinos, the prospects for detection of these neutrinos from future galactic CCSNe, and the effects these neutrinos can have on nucleosynthesis.

What can be learned from a proto-neutron star’s mass and radius?

“…It should be mentioned here, too, that the PNS is very likely unstable to convection, flattening the entropy per baryon profile in the shocked region (see e.g. the discussion in Roberts & Reddy 2017). This flattening is not reproduced by our profiles and the simulated PNS evolution does not treat convection.…”

“…It is in particular assumed here that the eventual supernova explosion is successful, so that matter outside the shock is excised from the numerical grid, once this shock has crossed the matter of the PNS (see the discussion e.g. by Roberts & Reddy 2017). The starting point is the expression for the metric of a static, spherically symmetric spacetime, using the standard Schwarzschild-type coordinates:…”

“…These profiles are then evolved using a quasi-static evolution code (Pascal et al (2021)): between two timesteps the electron fraction Ye and the entropy per baryon s are evolved using equations ( 7)-( 8) (see e.g. Pons et al (1999) and Roberts & Reddy (2017))…”

Gravitational wave signals in the deci-Hz range from neutrinos during the proto-neutron star cooling phase

“…Core-collapse supernovae (CCSNe) [1] are energetic explosions that occur at the final stage of massive stars to produce neutron stars as a result normally. At its birth just after the launch of an explosion, the neutron star is very hot, T 10 11 K, and still protonand lepton-rich, Y p ∼ Y e 0.2 [2]. It is hence referred to as the proto-neutron star (PNS) [3].…”

“…In these investigations, the GW emissions from neutrinos were indeed calculated quantitatively in addition to those from nonspherical matter motions, which will occur commonly via hydrodynamical instabilities, rotation, and magnetic stresses [11][12] [13][14] [15]. They found that the GWs from the neutrinos emitted anisotropically have the following properties: (1) their typical frequencies are lower than those of the GWs produced by matter, since the neutrino luminosities change more gradually; (2) the GW signal from neutrino has a so-called memory, i.e., the metric perturbation does not go back to zero after the passage of GW; in other words, the zero frequency limit of the GW signal is nonvanishing; (3) the GW strain may be dominated by the contribution from neutrinos at low frequencies although the energy carried by those GWs is minor. See [8][9] for more details.…”