The XRISM First Light Observation: Velocity Structure and Thermal Properties of the Supernova Remnant N132D

We present an initial analysis of the XRISM first-light observation of the supernova remnant (SNR) N132D in the Large Magellanic Cloud. The Resolve microcalorimeter has obtained the first high-resolution spectrum in the 1.6-10 keV band, which contains K-shell emission lines of Si, S, Ar, Ca, and Fe. We find that the Si and S lines are relatively narrow, with a broadening represented by a Gaussian-like velocity dispersion of σᵥ ~ 450 km s⁻¹. The Fe Heα lines are, on the other hand, substantially broadened with σᵥ ~ 1670 km s⁻¹. This broadening can be explained by a combination of the thermal Doppler effect due to the high ion temperature and the kinematic Doppler effect due to the SNR expansion. Assuming that the Fe Heα emission originates predominantly from the supernova ejecta, we estimate the reverse shock velocity at the time when the bulk of the Fe ejecta were shock heated to be -1000 Vᵣₛ~[ km s⁻¹] 3300 (in the observer frame). We also find that Fe Lyα emission is redshifted with a bulk velocity of ~ 890 km s⁻¹, substantially larger than the radial velocity of the local interstellar medium surrounding N132D. These results demonstrate that high-resolution X-ray spectroscopy is capable of providing constraints on the evolutionary stage, geometry, and velocity distribution of SNRs.