Discovery of the Highly-Neutronized Ejecta Clump with Enhanced Abundances of Titanium and Chromium in the Type Ia Supernova Remnant 3C 397

The supernova remnant (SNR) 3C 397 is thought to originate from a Type Ia supernova (SN Ia) explosion of a near-Chandrasekhar-mass ($M_{\rm Ch}$) progenitor, based on the enhanced abundances of Mn and Ni revealed by previous X-ray study with Suzaku. Here we report follow-up XMM-Newton observations of this SNR, conducted with the aim of investigating the detailed spatial distribution of the Fe-peak elements. We have discovered an ejecta clump with extremely high abundances of Ti and Cr, in addition to Mn, Fe, and Ni, in the southern part of the SNR. The Fe mass of this ejecta clump is estimated to be $\sim$ 0.06 $M_{\odot}$, under the assumption of a typical Fe yield for SNe Ia (i.e., $\sim$ 0.8 $M_{\odot}$). The observed mass ratios among the Fe-peak elements and Ti require substantial neutronization that is achieved only in the innermost regions of a near-$M_{\rm Ch}$ SN Ia with a central density of $\rho_c \sim 5 \times 10^9$ g cm$^{-3}$, significantly higher than typically assumed for standard near-$M_{\rm Ch}$ SNe Ia ($\rho_c \sim 2 \times 10^9$ g cm$^{-3}$). The overproduction of the neutron-rich isotopes (e.g., $^{50}$Ti and $^{54}$Cr) is significant in such high-$\rho_c$ SNe Ia, with respect to the solar composition. Therefore, if 3C 397 is a typical high-$\rho_c$ near-$M_{\rm Ch}$ SN Ia remnant, the solar abundances of these isotopes could be reproduced by the mixture of the high- and low-$\rho_c$ near-$M_{\rm Ch}$ and sub-$M_{\rm Ch}$ Type Ia events, with $\lesssim$ 20 % being high-$\rho_c$ near-$M_{\rm Ch}$.