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

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 (≳1) and high O viii Lyβ/Lyα ratio (≳0.2) found in multiepoch XMM-Newton Reflection Grating Spectrometer observations of SN 1987A. The line ratios cannot be fully explained by nonequilibrium 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 Lyα line and constrained the optical depth of the two lines as τO VII ∼ 0.6 and τO VIII ∼ 0.2. We estimated the temperature as kTe ∼ 0.15 keV and the oxygen column density as NO ∼0.5 × 1016 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 ∼20% compared with previous results. The N/O ratio by number of atoms is revised to be ∼1.2.