Gamma-Ray Echo as a Probe of Supernova Neutrino Emission Anisotropy and Long-Baseline Effects

During a core-collapse supernova (CCSN), the emitted electron antineutrino (ν e) burst initiates inverse beta decay (IBD) in the outer hydrogen layer of the stellar envelope. This leads to the production of a characteristic 511 keV photon signal, termed the Gamma-Ray Echo. The same neutrino burst, when detected on Earth in large neutrino observatories such as Hyper-Kamiokande, will offer information about the ν e content of the neutrino flux reaching Earth. In this work, we show that comparing the ν e content inferred from the gamma-ray echo with that inferred from terrestrial neutrino observations during a nearby CCSN event can offer a promising probe of large neutrino emission anisotropies and long-baseline propagation effects. In the latter case, the combined SN-Earth observations effectively act as a near-far detector configuration for neutrino propagation over an astrophysical baseline. We find that, for a near-Earth supernova, anisotropies or non-standard propagation effects at the level of tens of percent can be tested using gamma-ray telescopes with effective areas of O(10 4) cm squared. Therefore, our results motivate the development of the next generation of large effective area gamma-ray telescopes operating in the MeV gap.