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

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 $\delta m^2\gtrsim {10}^{-18}{\mathrm{e}\mathrm{V}}^2(0.22\mathrm{k}\mathrm{p}\mathrm{c}\mathrm{/}D)(E_{\star }\mathrm{/}15\mathrm{M}\mathrm{e}\mathrm{V})$. 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.