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 K alpha line of this SNR accurately. The ionization time-scale (tau) of the Fe ejecta was measured to be log(10)tau (cm(-3) s) = 10.24 +/- 0.03, significantly smaller than previous measurements. Marginally detected K alpha 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 +/- 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 similar to 20 kpc. The large ISM-to-ejecta shocked mass ratio of similar to 100 and dynamical time-scale of similar to 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.