High-Energy Neutrinos from Supernova Shock Breakouts in Circumstellar Media: Light Curves, Spectra, and Contribution to the Extragalactic Neutrino Background

Abstract

Enhanced mass loss from core-collapse supernova (SN) progenitors shortly before explosion appears to be common, creating a compact, optically thick circumstellar medium (CSM) at $\sim10^{14}-10^{15}$ cm. We derive an analytic description of the light curves and spectra of high-energy neutrinos emitted by nonrelativistic SN shock breakouts through such CSM, as a function of shock velocity and CSM parameters, accounting for the evolution of the hydrodynamic structure and the electromagnetic (EM) spectrum as the shock transitions from being radiation-mediated to collisionless. This evolution determines the time-dependent neutrino production efficiency, the maximum proton/neutrino energy, and the pair-production optical depth. A significant fraction of the neutrino energy is typically emitted within a few days of explosion, during breakout and before the EM light curve peak, with $1-100$ TeV neutrinos carrying $\approx10\%$ of the energy of shock-accelerated protons. The escape of high-energy photons ($>1$~GeV) is suppressed by pair-production for compact CSM configurations. If enhanced mass losses are common, and assuming that shock-accelerated protons carry $\approx10\%$ of the collisionless shock energy, CSM SN breakouts may significantly contribute to the observed high-energy neutrino background, without overproducing a corresponding high-energy gamma-ray background. SNe producing $>1$ neutrino events in a $1\left(10\right){\rm km^2}$ detector are expected at a rate of $\sim0.05\left(1\right){\rm yr^{-1}}$.