Discovery of recombining plasma in the supernova remnant 3C 391

Abstract

Recent X-ray study of middle-aged supernova remnants (SNRs) reveals strong radiative recombination continua (RRCs) associated with overionized plasmas, which origin still remains uncertain. We report a discovery of an RRC in the middle-aged SNR 3C∈391. If the X-ray spectrum is fitted with a two-temperature plasma model in collisional ionization equilibrium (CIE), residuals of a Si xiv Lyα line at 2.006 keV, a S xvi Lyα line at 2.623 keV and the edge of a RRC of Si xiii at 2.666 keV are found. In fact, the X-ray spectrum is well described by the composite model consisting of the CIE plasma and a recombining plasma (RP). The abundance pattern suggests that the RP is due to ejecta from core-collapse supernova of the progenitor mass of about 15∈M. There is no significant difference of the recombining parameters between the south-east region and the north-west region surrounded by dense molecular clouds. We also find a hint of a Fe i Kα line at 6.4 keV (2.4∈σ detection) from the south-east region of the SNR.