The Missing MeV Window for Identifying Galactic PeVatrons: the need for a dedicated MeV Observatory

Abstract

The identification of the sources responsible for accelerating Galactic cosmic-rays up to PeV energies remains a major open question in high-energy astrophysics. Recent observations by the Large High Altitude Air Shower Observatory (LHAASO) have revealed a growing population of Galactic PeVatron candidates. However, the detection of TeV-PeV gamma-rays alone does not uniquely identify the nature of the emitting particles. In many cases, both hadronic and leptonic scenarios can reproduce the observed very-high-energy emission, leaving the origin of the radiation unresolved. The most robust and least model-dependent electromagnetic tracer of hadronic acceleration is provided by neutral-pion decay, which produces the characteristic pion bump in the 10-100 MeV energy range. Observations in this energy band are therefore essential to establish whether candidate PeVatrons are accelerating protons and nuclei and, consequently, to assess their role in the origin of Galactic cosmic-rays. More broadly, the MeV domain addresses key astrophysical questions, but remains one of the least explored regions of the electromagnetic spectrum. As current and future VHE facilities expand the Galactic PeVatron census, the lack of MeV observations is becoming a major limitation for source identification. We discuss the astrophysical science drivers and observational requirements for a dedicated MeV gamma-ray capability optimized for pion-bump studies. We explore the concept of a compact, focused, and potentially fast-track mission capable of filling the impending MeV observational gap while more ambitious next-generation MeV facilities are developed and evaluated. Such a mission would provide the missing diagnostic needed to distinguish hadronic and leptonic emission, identify Galactic cosmic-ray accelerators, and fully exploit the scientific return of the emerging generation of TeV observatories.