  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.
