Unusual X-ray Oxygen Line Ratios of SN 1987A Arising From the Absorption of Galactic Hot Interstellar Medium

TL;DR

This study explains unusual X-ray oxygen line ratios in SN 1987A by foreground hot gas absorption, revising the oxygen abundance and N/O ratio, and highlights the importance of considering interstellar medium effects in spectral analysis.

Contribution

It introduces a new absorption model to explain anomalous line ratios, leading to revised elemental abundances in SN 1987A.

Findings

Foreground hot gas absorption explains line ratio anomalies.

Revised oxygen abundance increased by ~20%.

N/O ratio revised to ~1.2.

Abstract

Recent high-resolution X-ray spectroscopic studies have revealed unusual oxygen line ratios, such as the high O VII forbidden-to-resonance ratio, in several supernova remnants. While the physical origin is still under debate, for most of them, it has been suggested that this phenomenon arises from either charge exchange (CX) or resonant scattering (RS). In this work, we report the high O VII G-ratio ($\gtrsim1$) and high O VIII Ly$\beta$/Ly$\alpha$ ratio ($\gtrsim0.2$) found in multiepoch XMM-Newton RGS observations of SN 1987A. The line ratios cannot be fully explained by non-equilibrium ionization effects, CX, or RS. We suggest the absorption of foreground hot gas as the most likely origin, which plays the major role in modifying line fluxes and line ratios. Based on this scenario, we introduced two Gaussian absorption components at the O VII resonance line and the O VIII Ly$\alpha$ line and constrained the optical depth of the two lines as $\tau_{\rm OVII}\sim0.6$ and $\tau_{\rm OVIII}\sim0.2$. We estimated the temperature as $kT_{\rm e}\sim0.15$ keV and the oxygen column density as $N_{\rm O}\sim0.5\times10^{16}$ cm$^{-2}$ for the absorbing gas, which is consistent with the hot interstellar medium in the Galactic halo. Neglecting this absorption component may lead to an underestimation of the O abundance. We revised the O abundance of SN 1987A, which is increased by $\sim20\%$ compared with previous results. The N/O ratio by number of atoms is revised to be $\sim1.2$.