Recombining Plasma in the Gamma-ray Emitting Mixed-Morphology Supernova Remnant 3C 391

TL;DR

This paper reports the detection of gamma-ray emission from the supernova remnant 3C 391, revealing overionized plasma and suggesting hadronic processes as the origin of the gamma rays, contributing to understanding mixed-morphology SNRs interacting with molecular clouds.

Contribution

First detection of GeV gamma rays from 3C 391 and identification of overionized plasma with implications for gamma-ray origin in mixed-morphology SNRs.

Findings

3C 391 detected in GeV gamma rays with ~18 sigma significance

Gamma-ray spectrum explained by neutral pion decay from proton interactions

Recombination structures of silicon and sulfur observed in X-ray data

Abstract

A group of middle-aged mixed-morphology (MM) supernova remnants (SNRs) interacting with molecular clouds (MC) has been discovered as strong GeV gamma-ray emitters by Large Area Telescope on board Fermi Gamma Ray Space Telescope (Fermi-LAT). The recent observations of the Suzaku X-ray satellite have revealed that some of these interacting gamma-ray emitting SNRs, such as IC443, W49B, W44, and G359.1-0.5, have overionized plasmas. 3C 391 (G31.9+0.0) is another Galactic MM SNR interacting with MC. It was observed in GeV gamma rays by Fermi-LAT as well as in the 0.3 $-$ 10.0 keV X-ray band by Suzaku. In this work, 3C 391 was detected in GeV gamma rays with a significance of $\sim$ 18 $\sigma$ and we showed that the GeV emission is point-like in nature. The GeV gamma-ray spectrum was shown to be best explained by the decay of neutral pions assuming that the protons follow a broken power-law distribution. We revealed radiative recombination structures of silicon and sulfur from 3C 391 using Suzaku data. In this paper we discuss the possible origin of this type of radiative plasma and hadronic gamma rays.