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.

Comparing Neutron Star Kicks to Supernova Remnant Asymmetries

“…The origin of these asymmetries remains unclear: they may reflect the asymmetries inherent to the explosion mechanism, or they may arise from interactions with an inhomogeneous medium. For example, kinematic studies demonstrate large-scale asymmetries in SNR ejecta (e.g., SN 1987A: Boggs et al 2015; Cas A: Rest et al 2011;Grefenstette et al 2017), while observations and simulations show that interaction with a dense medium can alter the morphology and thermodynamic properties of SNRs (e.g., Tenorio-Tagle et al 1985;Lazendic & Slane 2006;Slane et al 2015).…”

Molecular Environments of Three Large Supernova Remnants in the Third Galactic Quadrant: G205.5+0.5, G206.9+2.3, and G213.0–0.6

“…So far, there are several signposts to show the physical contact/interaction between SNRs and their molecular environments, e.g., the presence of 1720 MHz OH maser emission, molecular line broadenings or asymmetric profiles, high molecular line ratios between different excitation states, detection of H 2 and/or [Fe II] lines in near-infrared, specific infrared colors within an SNR, and often used morphological correspondence in multiwavelength with SNR features (see Jiang et al 2010). It is also important to study the interplay of SNR-MC systems for investigating various physical and astrophysical processes therein (e.g., Chen et al 2014;Dubner & Giacani 2015;Slane et al 2015).…”

An XMM-Newton Study of the Mixed-morphology Supernova Remnant G346.6-0.2

“…The first one is the thermal conduction model in which heat and material are transported to the centre of the remnant via the Coulomb collisions between electrons and ions inside the hot plasma, resulting in the centrally-filled emission and isothermal temperatures (Cui & Cox 1992;Cox et al 1999). The sec-8 Even though there are a number of methods (see Slane et al 2015 for more details) used to infer the presence of SNR/MC interaction, the detection of an OH maser is a "smoking gun" signal, since they can only be formed in conditions related to a shock/molecular cloud interaction. A handful of these non-thermal SNRs such as RX J1713.7−3946 (Slane et al 1999;Butt et al 2001) show evidence of shock interaction in the form of other molecular line features, however none of these sources so far show evidence of an OH maser.…”