The origin of Cosmic-Rays from SNRs: confirmations and challenges after the first direct proof

TL;DR

This paper reviews evidence supporting supernova remnants as sources of cosmic rays, highlighting recent gamma-ray observations of SNR W44 that confirm pion emission and discuss remaining theoretical challenges.

Contribution

It provides a comprehensive reassessment of data and models for SNR W44, strengthening evidence for hadronic cosmic-ray acceleration and identifying ongoing theoretical issues.

Findings

Gamma-ray continuum below 200 MeV attributed to neutral pion emission

Confirmation of hadronic origin of gamma-ray emission in W44

Identification of theoretical challenges in cosmic-ray acceleration models

Abstract

Until now, providing an experimental unambiguous proof of Cosmic Ray (CR) origin has been elusive. The SuperNova Remnant (SNR) study showed an increasingly complex scenario with a continuous elaboration of theoretical models. The middle-aged supernova remnant (SNR) W44 has recently attracted attention because of its relevance regarding the origin of Galactic cosmic-rays. The gamma-ray missions AGILE and Fermi have established, for the first time for a SNR, the spectral continuum below 200 MeV which can be attributed to neutral pion emission. Our work is focused on a global re-assessment of all available data and models of particle acceleration in W44 and our analysis strengthens previous studies and observations of the W44 complex environment, providing new information for a more detailed modeling. However, having determined the hadronic nature of the gamma-ray emission on firm ground, a number of theoretical challenges remains to be addressed in the context of CR acceleration in SNRs.