  Despite tantalizing evidence that supernova remnants (SNRs) are the source of
Galactic cosmic rays (CRs), including the recent detection of a spectral
signature of hadronic gamma-ray emission from two SNRs, their origin in
aggregate remains elusive. We address the long-standing question of Galactic CR
nuclei origins using our statistically significant GeV SNR sample to estimate
the contribution of SNRs to directly observed CRs. Interactions between CRs and
ambient gas near the SNRs emit photons via pion decay at GeV energies,
providing an in situ tracer for CRs otherwise measured directly with
balloon-borne and satellite experiments near the Earth. To date, the Fermi LAT
SNR Catalog has detected more than 50 SNRs and potential associations in
classes with a variety of properties, yet all remain possible accelerators. We
investigate the GeV and multiwavelength (MW) emission from SNRs to constrain
their maximal contribution to observed Galactic CRs. Our work demonstrates the
need for improvements to previously sufficient simple models describing the GeV
and MW emission from these objects.

High Energy Astrophysical Phenomena