The low rate of Galactic pevatrons

TL;DR

This paper investigates whether supernova remnants can accelerate cosmic rays to the knee energy, finding only rare, powerful supernovae can reach these energies, impacting our understanding of Galactic cosmic ray sources.

Contribution

It provides a detailed calculation of particle spectra from various supernova remnants, highlighting the rarity of remnants capable of reaching PeV energies and constraining their contribution to Galactic cosmic rays.

Findings

Only powerful, rare core-collapse supernovae can accelerate particles to the knee.

Typical core-collapse supernovae reach maximum energies too late, producing steep spectra at high energies.

Type Ia supernovae can only produce cosmic rays up to about 10^5 GeV.

Abstract

Although supernova remnants remain the main suspects as sources of Galactic cosmic rays up to the knee, the supernova paradigm still has many loose ends. The weakest point in this construction is the possibility that individual supernova remnants can accelerate particles to the rigidity of the knee, $\sim 10^{6}$ GV. This scenario heavily relies upon the possibility to excite current driven non-resonant hybrid modes while the shock is still at the beginning of the Sedov phase. These modes can enhance the rate of particle scattering thereby leading to potentially very-high maximum energies. Here we calculate the spectrum of particles released into the interstellar medium from the remnants of different types of supernovae. We find that only the remnants of very powerful, rare core-collapse supernova explosions can accelerate light elements such as hydrogen and helium nuclei, to the knee rigidity, and that the local spectrum of cosmic rays directly constrains the rate of such events, if they are also source of PeV cosmic rays. On the other hand, for remnants of typical core-collapse supernova explosions, the Sedov phase is reached at late times, when the maximum energy is too low and the spectrum at very-high energies is very steep, being mostly produced during the ejecta dominated phase. For typical thermonuclear explosions, resulting in type Ia supernovae, we confirm previous findings that these objects can only produce cosmic rays up to $\lesssim 10^{5}$ GeV. The implications for the overall cosmic ray spectrum observed at the Earth and for the detection of PeVatrons by future gamma-ray observatories are discussed.