The Potential Contribution of Radiative Supernova Remnants to the Local Cosmic Ray Spectrum

Abstract

Recent observations of very-high-energy gamma rays from massive stellar clusters and microquasars have demonstrated that these systems can efficiently accelerate particles to very high energies, renewing the debate on the dominant sources of Galactic cosmic rays (CRs) and casting further doubt on the role of supernova remnants (SNRs), long considered the prime candidates. In these astrophysical environments, particle acceleration is generally thought to be driven by diffusive shock acceleration (DSA). The question of the duration of the phase in which DSA can efficiently take place is therefore essential to clarify the contribution of the various candidate CR sources. We aim at clarifying the contribution of SNRs to the Galactic CR spectrum, and the expected efficiency of DSA, especially accounting for possible particle acceleration during part of the radiative phase. We compute the total CR proton content at typical SNRs from DSA and reaccelerated pre-existing CRs, and discuss the importance of these two mechanisms throughout the adiabatic and early radiative phases. We find that if DSA and reacceleration take place during a fraction of the radiative phase, the total content of particles energized at SNRs can quickly overcome the typical budget required to account for the bulk of CRs. This suggests that the typical instantaneous efficiency, often defined as the fraction of fluid ram pressure converted into CRs would need to remain below $\lesssim 1$\% for most of the SNR evolution. Moreover, the radiative phase can contribute to shaping the spectra injected from typical SNRs. We illustrate that the difference between SNRs from thermonuclear and core-collapse SNe could help account for the measured difference in the helium and proton of the local interstellar spectrum.