Precise measurement of pion-bump structure using future MeV gamma-ray detectors

The pion-bump structure in the $\ensuremath{\gamma}$-ray spectrum is a direct proof for the hadronic origin of the $\ensuremath{\gamma}$ rays, and thus the decisive evidence for the acceleration of hadronic cosmic rays in astrophysical objects. However, the identification of such a spectral feature is limited by the resolution and energy coverage of current $\ensuremath{\gamma}$-ray instruments. Furthermore, there are unavoidable bremsstrahlung emissions from secondary and primary electrons, which may dominate the $\ensuremath{\gamma}$-ray emission below the pion-bump. Thus, the study of this $\ensuremath{\gamma}$-ray emission component can provide unique information on the acceleration and confinement of high-energy particles. In this paper, we studied the predicted $\ensuremath{\gamma}$-ray spectrum assuming both hadronic or leptonic origin in mid-aged supernova remnants W44, we discuss the detection potential of future MeV missions on these emissions and possible implications.