Supernova Gamma-Ray Echo as an Astrophysical Near Detector for Galactic Neutrino Propagation

Abstract

Core-collapse supernova neutrinos offer a unique probe of physics beyond the Standard Model over Galactic baselines. Yet even a high-statistics nearby burst may be limited by source uncertainties, especially for propagation effects that average over energy and appear only as an overall flux suppression. Inspired by near--far comparisons in terrestrial oscillation experiments, we show that next-generation MeV observations of the supernova gamma-ray echo---the delayed 511 keV signal induced by the neutrino burst in the stellar envelope---can serve as an astrophysical near detector for the emitted $\bar\nu_e$ fluence. Comparing this echo-inferred source fluence with the surviving flux measured at Earth gives a direct test of neutrino propagation between the stellar surface and the detector. For pseudo-Dirac neutrinos, this turns the fully averaged active--sterile oscillation regime from an unobservable source-normalization ambiguity into a measurable near--far deficit, with sensitivity for nearby progenitors to splittings $\delta m^2\gtrsim 10^{-18}~{\rm eV}^2(0.22~{\rm kpc}/D)(E_\star/15~{\rm MeV})$. The same near--far logic also applies to other propagation-induced modifications of the observed flux, including neutrino decay and neutrino interactions with Galactic dark matter.