Overionized plasma in the supernova remnant Sagittarius A East anchored by XRISM observations

Sagittarius A East is a supernova remnant with a unique surrounding environment, as it is located in the immediate vicinity of the supermassive black hole at the Galactic center, Sagittarius A^∗. The X-ray emission of the remnant is suspected to show features of overionized plasma, which would require peculiar evolutionary paths. We report on the first observation of Sagittarius A East with the X-Ray Imaging and Spectroscopy Mission (XRISM). Equipped with a combination of a high-resolution microcalorimeter spectrometer and a large field-of-view CCD imager, we for the first time resolved the Fe XXV K-shell lines into fine structure lines and measured the forbidden-to-resonance intensity ratio to be 1.39 ± 0.12, which strongly suggests the presence of overionized plasma. We obtained a reliable constraint on the ionization temperature just before the transition into the overionization state, of >4 keV. The recombination timescale was constrained to be <8 × 10¹¹ cm⁻³ s. The small velocity dispersion of 109 ± 6 km s⁻¹ indicates a low Fe ion temperature <8 keV and a small expansion velocity <200 km s⁻¹. The high initial ionization temperature and small recombination timescale suggest that either rapid cooling of the plasma via adiabatic expansion from dense circumstellar material or intense photoionization by Sagittarius A^∗ in the past may have triggered the overionization.