Speaker
Description
During a core-collapse supernova (CCSN), the electron anti-neutrino ($\bar{\nu}_e$) signal initiates inverse beta decay (IBD) in the outer hydrogen layer of the stellar envelope. This leads to the production of a characteristic 511 keV signal, termed the \emph{Gamma-Ray Echo}. The same neutrino signal, when detected on Earth in large neutrino observatories such as Hyper-K and IceCube, provides complementary information about the $\bar{\nu}_e$ content of the neutrino burst. In this work, we show that comparing the $\bar{\nu}_e$ content inferred from the gamma-ray echo with that inferred from terrestrial neutrino observations during a nearby CCSN event provides a powerful probe of large neutrino emission anisotropies as well as long-baseline flavor oscillation effects. In the latter case, the combined supernova--Earth observations effectively act as a calibrated near--far detector configuration for neutrino propagation over astrophysical baselines. Such a measurement would also enable an independent normalization of the supernova neutrino flux, thereby reducing key systematic uncertainties in the interpretation of future Galactic supernova neutrino observations and strengthening the physics reach of next-generation neutrino detectors. We therefore strongly emphasize the need for large effective-area telescopes capable of sensitive 511 keV observations.