Ultra-high-energy $\gamma$-ray imprints from PeV particles accelerated by supernova remnants

TL;DR

This study presents the first evidence of PeV cosmic ray acceleration in supernova remnants, demonstrated by very-high-energy gamma-ray emissions up to hundreds of TeV from two SNRs observed with LHAASO.

Contribution

It provides observational evidence that SNRs can accelerate particles beyond PeV energies, supporting their role as sources of the highest energy Galactic cosmic rays.

Findings

Detection of gamma-ray emission up to hundreds of TeV from two SNRs.

Identification of multiple spectral components with convex shapes.

Association of high-energy emission with molecular clouds, indicating hadronic CR interactions.

Abstract

The quest for the origin of cosmic ray (CRs) is a fundamental issue in astrophysics. Shocks of supernova remnants (SNRs) have been considered as the dominant contributors to Galactic CRs below the spectral knee near $\sim 3$ petaelectronvolt (PeV). Whether SNRs are efficient accelerators of particles beyond PeV energies has long been debated. Here we report observations of very-high-energy $\gamma$-ray emission up to hundreds of TeV from two middle age shell-type SNRs, G150.3$+$4.5 and $\gamma$-Cygni, with the Large High Altitude Air Shower Observatory (LHAASO). Two (or three) distinct morphological/spectral components with convex spectral shapes are observed in both sources, with the low-energy one being more extended than the high-energy one. %Although it is possible that these high-energy components may be driven by powerful pulsars, The likely association of the high-energy component with molecular clouds at similar distances, and the weakness/absence of pulsar wind nebulae (PWNe) inside these SNRs clearly indicate for the first time that the highest energy emission is produced by collision of hadronic CRs up to PeV energies with the clouds. These results are compatible with the classic model prediction that PeV particles accelerated near the end of the free expansion phase of SNR evolution can illuminate nearby molecular clouds (MCs) to produce strong $\gamma$-ray emission.