Limits on compression of cosmic rays in supernova remnants

TL;DR

This paper critically examines the scenario of cosmic ray compression in supernova remnants, showing that observational data strongly limit the extent of compression and challenge previous models of gamma-ray production.

Contribution

It provides a detailed analysis demonstrating that the gamma-ray spectra and radio observations impose strict limits on cosmic ray compression in supernova remnants.

Findings

Compressed GCRs cannot produce enough gamma-rays to match observations.

Absence of spectral curvature in radio spectra disfavor the compression scenario.

Compressed electrons contribute at most ~10% to radio flux.

Abstract

The spectral shape of the gamma-ray emission observed for dynamically old supernova remnants that interact with molecular clouds triggered an exciting scenario of adiabatic compression and farther re-acceleration of Galactic cosmic rays (GCRs) in radiative shells of the remnants, which was extensively discussed and applied to various sources over recent years. Indeed, the observed gamma-ray spectrum from a number of remnants strongly resembles the expected spectrum of the gamma-ray emission from the compressed population of Galactic cosmic rays. In the following we discuss the feasibility of this scenario and show that it is very unlikely that compressed GCRs could produce sufficient amount of gamma-rays and that the observed spectral shape is putting strong limits on the allowed compression factors. Further, absence of curvature in featureless power-law spectra of evolved supernova remnants at radio wavelengths is strongly disfavoring the compression scenario for electrons and hence for hadrons. Our calculations show that the contribution of compressed electrons to the observed radio-flux could reach at most ~10%.