Supernova Remnants Interacting with Molecular Clouds: X-Ray and Gamma-Ray Signatures

Abstract

The giant molecular clouds (MCs) found in the Milky Way and similar galaxies play a crucial role in the evolution of these systems. The supernova explosions that mark the death of massive stars in these regions often lead to interactions between the supernova remnants (SNRs) and the clouds. These interactions have a profound effect on our understanding of SNRs. Shocks in SNRs should be capable of accelerating particles to cosmic ray (CR) energies with efficiencies high enough to power Galactic CRs. X-ray and γ-ray studies have established the presence of relativistic electrons and protons in some SNRs and provided strong evidence for diffusive shock acceleration as the primary acceleration mechanism, including strongly amplified magnetic fields, temperature and ionization effects on the shock-heated plasmas, and modifications to the dynamical evolution of some systems. Because protons dominate the overall energetics of the CRs, it is crucial to understand this hadronic component even though electrons are much more efficient radiators and it can be difficult to identify the hadronic component. However, near MCs the densities are sufficiently high to allow the γ-ray emission to be dominated by protons. Thus, these interaction sites provide some of our best opportunities to constrain the overall energetics of these particle accelerators. Here we summarize some key properties of interactions between SNRs and MCs, with an emphasis on recent X-ray and γ-ray studies that are providing important constraints on our understanding of cosmic rays in our Galaxy.

Observations of supernova remnants and pulsar wind nebulae at gamma-ray energies

“…Excellent gamma-ray targets are provided by dense regions of the interstellar medium or in some cases dense molecular clouds interacting with middle-age SNRs (∼ 10 11 -10 12 s or 3,000-30,000 yr). For more information, see the recent review [58]. They are the most numerous subclass of GeV SNRs due to enhanced target densities for accelerated particles, which in cases of interaction with a molecular cloud can increase gamma-ray luminosities by factors of 10 2 − 10 3 .…”

Similarity of γ -ray spectrum in middle aged supernova remnants (SNRs) interacting with molecular clouds (MC): what can we learn?

“…In the past few years, both space-based GeV observatories (Fermi and AGILE) and groundbased TeV observatories (H.E.S.S, MAGIC and VERITAS) detect γ-ray emission from several middle aged supernova remnants (SNRs) which are interacting with molecular clouds (hereafter SNR/MC) [1]. Multi-wavelength observations further demonstrate that the γ-ray emission region is spatially correlated with the MC interaction region [2]. The detected γ-ray emission is produced either by energetic electrons with Bremsstrahlung and Inverse Compton (IC) emission mechanism or accelerated protons with π 0 -decay emission mechanism through proton-proton collision.…”

Negative and Positive Feedback from a Supernova Remnant with SHREC: A detailed Study of the Shocked Gas in IC443

“…These sources have been selected for being relatively nearby (kinematic distance ≤ 6 kpc) and for showing evidence of interaction with the surrounding molecular material (Ferrand & Safi-Harb 2012;Green 2019). This includes enhanced X-ray emission, the presence of OH maser emission at 1720 MHz and enhanced CO(2-1)/CO(1-0) ratios (Slane et al 2016). The final goal of SHREC is to identify sites of large-scale interactions driven by SNRs and to investigate how these affect the star formation potential and dispersal of the surrounding molecular material.…”