The Transition from Young to Middle-aged Supernova Remnants: Thermal and Nonthermal Aspects of SNR N132D

Supernova remnants (SNRs) are the primary candidate of Galactic cosmic-ray accelerators. It is still an open issue when and how young SNRs, which typically exhibit strong synchrotron X-rays and GeV and TeV gamma rays, undergo the state transition to middle-aged SNRs dominated by thermal X-rays and GeV gamma rays. SNR N132D in the Large Magellanic Cloud is an ideal target to study such a transition, exhibiting bright X-rays and gamma rays, and with an expected age of ∼2500 years. In this paper we present results of NuSTAR and Suzaku spectroscopy. We reveal that N132D has a nearly equilibrium plasma with a temperature of >5 keV or a recombining plasma with a lower temperature (∼1.5 keV) and a recombining timescale ({n}_et) of 8.8 (7.0{--}10.0)× {10}¹² cm^-3s. Together with the center-filled morphology observed in the iron K line image, our results suggest that N132D is now at the transition stage from being a young SNR to being middle-aged. We have constrained the tight upper limit of nonthermal X-rays. Bright gamma rays compared to faint nonthermal X-rays suggest that the gamma rays are hadronic in origin. The spectral energy distribution from radio to gamma rays shows a proton cutoff energy of ∼30 TeV. These facts confirm that N132D is undergoing the transition from a young to a middle-aged SNR. The large thermal energy of > {10}⁵¹ erg and accelerated proton energy of ∼ {10}⁵⁰ erg suggest the supernova explosion might have been very energetic.