Evidence of cosmic-ray acceleration up to sub-PeV energies in the supernova remnant IC 443

TL;DR

This study presents evidence that supernova remnant IC 443 accelerates cosmic rays up to sub-PeV energies, based on high-energy gamma-ray observations indicating particles reaching hundreds of TeV.

Contribution

It provides the first observational evidence that SNRs can accelerate cosmic rays up to sub-PeV energies, extending previous knowledge of cosmic-ray acceleration limits.

Findings

Gamma-ray emission from IC 443 extends beyond 30 TeV.

Protons are accelerated to at least 300 TeV assuming hadronic origin.

Evidence supports SNRs as sources of sub-PeV cosmic rays.

Abstract

Supernova remnants (SNRs) have been considered as the primary contributors to cosmic rays (CRs) in our Galaxy. However, the maximum energy of particles that can be accelerated by shocks of SNRs is uncertain, and SNRs' contribution to CRs around PeV energies is unclear. In this study, we present observations of high-energy $\gamma$-ray emission from the SNR IC 443 using the Large High Altitude Air Shower Observatory (LHAASO). The morphological analysis reveals a pointlike source whose location and spectrum are consistent with those of the Fermi-LAT-detected compact source with $\pi^0$-decay signature, and a more extended source that is associated with a newly discovered Fermi source. The spectrum of the point source can be described by a power-law function with an index of $\sim3.0$, extending beyond $\sim 30$ TeV without apparent cutoff. Assuming a hadronic origin of the $\gamma$-ray emission, the $95\%$ lower limit of accelerated protons reaches about 300 TeV. The extended source might be associated with IC 443, SNR G189.6+3.3 or the putative pulsar wind nebula CXOU J061705.3+222127, and can be explained by either a hadronic or a leptonic model with particles reaching hundreds of TeV. These LHAASO results provide compelling evidence that sub-PeV CRs can be accelerated efficiently by shocks of SNRs.