Latest Evolution of the X-ray Remnant of SN 1987A: Beyond the Inner Ring

Based on our Chandra HETG and ACIS imaging-spectroscopic observations, we present the latest evolution of the X-ray remnant of SN 1987A. Recent changes in electron temperatures and volume emission measures suggest that the shocks moving through the inner ring have started interacting with less dense circumstellar material, probably beyond the inner ring. We find significant changes in the X-ray line flux ratios (among H- and He-like Si and Mg ions) in 2018, consistent with changes in the thermal conditions of the X-ray emitting plasma that we infer based on the broadband spectral analysis. Post-shock electron temperatures suggested by line flux ratios are in the range ~0.8 - 2.5 keV as of 2018. The soft-band (0.5 - 2.0 keV) X-ray flux has been on a slight declining trend (~10%) between 2018 - 2021, while the hard-band counterpart (3.0 - 8.0 keV) has been linearly increasing by ~10% yr^-1. We find that the overall expansion rate of the X-ray emitting ring still dominated by the bright inner ring has stayed constant (~1600 km s^-1) since the shock entered the main body of the inner ring in 2004. However, we note that after 2018, the expansion rate of the outer boundary of the X-ray emission, previously indistinguishable from the overall rate, has significantly increased (~4000 km s^-1), independently implying a low density CSM interaction with the shock front. We do not yet observe any evidence of substantial abundance enhancement, suggesting that the X-ray emission component from the reverse-shocked metal-rich ejecta is not yet significant in the observed X-ray spectrum. We also do not yet detect X-ray radiation from the compact source at the center in the 0.5 - 8.0 keV energy range.