Supernova remnant W44: a case of cosmic-ray reacceleration

Supernova remnants (SNRs) are thought to be the primary sources of Galactic cosmic rays (CRs). In the last few years, the wealth of gamma-ray data collected by GeV and TeV instruments has provided important information about particle energization in these astrophysical sources, allowing us to make progress in assessing their role as CR accelerators. In particular, the spectrum of the gamma-ray emission detected by AGILE and Fermi-LAT from the two middle-aged SNRs W44 and IC 443, has been proposed as a proof of CR acceleration in SNRs. Here we discuss the possibility that the radio and gamma-ray spectra from W44 may be explained in terms of reacceleration and compression of Galactic CRs. The recent measurement of the interstellar CR flux by Voyager 1 has been instrumental for our work, in that the result of the reprocessing of CRs by the shock in W44 depends on the CR spectrum at energies that are precluded from terrestrial measurement owing to solar modulation. We introduce both CR protons and helium nuclei in our calculations, and secondary electrons produced in situ are compared with the flux of Galactic CR electrons reprocessed by the slow shock of this SNR. We find that the multiwavelength spectrum of W44 can be explained by reaccelerated particles with no need of imposing any break on their distribution, but just a high-energy cutoff at the maximum energy the accelerator can provide. We also find that a model including both reacceleration and a very small fraction of freshly accelerated particles may be more satisfactory on physical grounds.