A Chandra X-ray study of the mixed-morphology supernova remnant 3C 400.2

Abstract

We present an analysis of archival Chandra observations of the mixed-morphology remnant 3C 400.2. We analysed spectra of different parts of the remnant to observe if the plasma properties provide hints on the origin of the mixed-morphology class. These remnants often show overionization, which is a sign of rapid cooling of the thermal plasma, and supersolar abundances of elements which is a sign of ejecta emission. Our analysis shows that the thermal emission of 3C 400.2 can be well explained by a two-component non-equilibrium ionization model, of which one component is underionized, has a high temperature (kT ≈ 3.9 keV) and supersolar abundances, while the other component has a much lower temperature (kT ≈ 0.14 keV), solar abundances and shows signs of overionization. The temperature structure, abundance values and density contrast between the different model components suggest that the hot component comes from ejecta plasma, while the cooler component has an interstellar matter origin. This provides the first evidence of an overionized plasma found in the outer regions of a supernova remnant, whereas the ejecta component of the inner region is underionized. In addition, the non-equilibrium ionization plasma component associated with the ejecta is confined to the central, brighter parts of the remnant, whereas the cooler component is present mostly in the outer regions. Therefore, our data can most naturally be explained by an evolutionary scenario in which the outer parts of the remnant are cooling rapidly due to having swept-up high-density ambient medium, while the inner parts are very hot and cooling inefficiently due to low density of the plasma. This is also known as the relic X-ray scenario.