Ionization age of iron ejecta in the Galactic Type Ia supernova remnant G306.3−0.9

We present a 190 ks observation of the Galactic supernova remnant (SNR) G306.3−0.9 with Suzaku. To study ejecta properties of this possible Type Ia SNR, the absolute energy-scale at the Fe-K band was calibrated to a level of uncertainty less than 10 eV by a cross-calibration with the Hitomi microcalorimeter using the Perseus cluster spectra. This enabled us for the first time to determine the ionization state of the Fe Kα line of this SNR accurately. The ionization time-scale (τ) of the Fe ejecta was measured to be log10τ (cm−3 s) $= 10.24\, \pm \, 0.03$, significantly smaller than previous measurements. Marginally detected Kα lines of Cr and Mn have ionization time-scales consistent with that of Fe. The global spectrum was well-fitted with shocked interstellar matter (ISM) and at least two ejecta components with different ionization time-scales for Fe and intermediate mass elements (IME) such as S and Ar. One plausible interpretation of the one-order-of-magnitude shorter time-scale of Fe than that of IME ($\log _{10} \tau = 11.17\, \pm \, 0.07$) is a chemically stratified structure of ejecta. By comparing the X-ray absorption column to the H i distribution decomposed along the line of sight, we refined the distance to ∼20 kpc. The large ISM-to-ejecta shocked mass ratio of ∼100 and dynamical time-scale of ∼6 kyr place the SNR in the late Sedov phase. These properties are consistent with a stratified ejecta structure that has survived the mixing processes expected in an evolved SNR.