Hitomi Observations of the LMC SNR N132D: Highly Redshifted X-ray Emission from Iron Ejecta

TL;DR

Hitomi observations of the LMC supernova remnant N132D reveal highly redshifted iron ejecta, demonstrating the capability of high-resolution X-ray spectroscopy to measure velocities and asymmetries in supernova remnants even with limited data.

Contribution

First high-resolution spectral measurement of Fe K emission in N132D, showing ejecta velocity and asymmetry, and demonstrating the effectiveness of short observations for velocity diagnostics.

Findings

Fe ejecta are highly redshifted at ~800 km/s.

Fe emission originates from supernova ejecta, not swept-up ISM.

High spectral resolution enables velocity measurements with few counts.

Abstract

We present Hitomi observations of N132D, a young, X-ray bright, O-rich core-collapse supernova remnant in the Large Magellanic Cloud (LMC). Despite a very short observation of only 3.7 ks, the Soft X-ray Spectrometer (SXS) easily detects the line complexes of highly ionized S K and Fe K with 16-17 counts in each. The Fe feature is measured for the first time at high spectral resolution. Based on the plausible assumption that the Fe K emission is dominated by He-like ions, we find that the material responsible for this Fe emission is highly redshifted at ~800 km/s compared to the local LMC interstellar medium (ISM), with a 90% credible interval of 50-1500 km/s if a weakly informative prior is placed on possible line broadening. This indicates (1) that the Fe emission arises from the supernova ejecta, and (2) that these ejecta are highly asymmetric, since no blue-shifted component is found. The S K velocity is consistent with the local LMC ISM, and is likely from swept-up ISM material. These results are consistent with spatial mapping that shows the He-like Fe concentrated in the interior of the remnant and the S tracing the outer shell. The results also show that even with a very small number of counts, direct velocity measurements from Doppler-shifted lines detected in extended objects like supernova remnants are now possible. Thanks to the very low SXS background of ~1 event per spectral resolution element per 100 ks, such results are obtainable during short pointed or slew observations with similar instruments. This highlights the power of high-spectral-resolution imaging observations, and demonstrates the new window that has been opened with Hitomi and will be greatly widened with future missions such as the X-ray Astronomy Recovery Mission (XARM) and Athena.