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

## TL;DR

This study uses high-precision X-ray spectroscopy to measure the ionization age of iron ejecta in the supernova remnant G306.3-0.9, revealing a stratified ejecta structure and providing insights into its evolution and distance.

## Contribution

First accurate measurement of Fe K$eta$ ionization timescale in G306.3-0.9 using Suzaku and Hitomi calibration, supporting a stratified ejecta model in an evolved SNR.

## Key findings

- Ionization timescale of Fe ejecta is $ au = 10^{10.24}$ cm$^{-3}$ s.
- Fe ejecta has a shorter ionization timescale than intermediate mass elements.
- SNR G306.3-0.9 is in late Sedov phase, approximately 6 kyr old.

## Abstract

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 accurately determine the ionization state of the Fe K$\alpha$ line of this SNR. The ionization timescale ($\tau$) of the Fe ejecta was measured to be $\log_{10} \tau$ (cm$^{-3}$ s) $=10.24\pm0.03$, significantly smaller than previous measurements. Marginally detected K$\alpha$ lines of Cr and Mn have consistent ionization timescales with Fe. The global spectrum was well fitted with shocked interstellar matter (ISM) and at least two ejecta components with different ionization timescales for Fe and intermediate mass elements (IME) such as S and Ar. One plausible interpretation of the one-order-of-magnitude shorter timescale of Fe than that of IME ($\log_{10} \tau = 11.17\pm0.07$) is a chemically stratified structure of ejecta. By comparing the X-ray absorption column to the HI distribution decomposed along the line of sight, we refined the distance to be $\sim$20 kpc. The large ISM-to-ejecta shocked mass ratio of $\sim$100 and dynamical timescale of $\sim$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 supernova remnant.