© NASA, ESA, and the Hubble SM4 ERO TeamOptical image of supernova remnant N132D in the nearby Large Magellanic Cloud.
Retracing Supernova Remnants
Massive stars, about 8 solar masses or larger, will end their short lives in violent core-collapse supernovae. These explosions barrel into any surrounding interstellar medium, carving out low-density cavities and blowing material outward. Appearing as a bubble or shell of hot gas, a supernova remnant carries the signatures of the type of star that produced it and the ways that star shaped its environment throughout its life.
Nearby, in the Large Magellanic Cloud, is the 2,500-year-old supernova remnant N132D — the most X-ray luminous supernova remnant within the Local Group. Though N132D's size, likely progenitor mass, and chemical composition are well constrained, astronomers have yet to nail down the velocity of the X-ray shock front — the outer edge of the supernova remnant that rams into the interstellar medium. This measurement is critical to understanding the local conditions the supernova first encountered and how its expansion will continue to impact the interstellar medium over time.
© Long et al 2025The expansion of supernova remnant N132D around the rim for each of the regions analyzed in this study. The arrow lengths are proportional to their estimated expansion velocities.
Determining the shock front velocity requires astronomers to focus on the thin outer edge of the soaring supernova remnant — how do we measure the motion of such a narrow strip of gas? X-ray spectroscopy of N132D yields a measurement from a single epoch of observations, but it cannot isolate the narrow shock front, making a reliable velocity measurement difficult to attain.
Circumventing the challenges of spectral analyses, Xi Long (University of Hong Kong) and collaborators used two sets of X-ray observations of N132D from the Chandra X-ray Observatory taken about 14.5 years apart to measure the motion of the shock front across the sky over time. This measurement, known as proper motion, compares the location of the foreground supernova remnant to stationary background stars to estimate its angular speed.
The authors focus on small regions on the very edge of the supernova shock front, six northern and eight southern, to carefully measure the motion of the shock between the two sets of observations. In separating the shock front into smaller regions, the authors can determine any variations in speed along the shock front. The southern edge of N132D moves with the same expansion rate of 1,620 km/s. The northern edge has an average expansion rate of 3,820 km/s, but its speed is more varied, which isn't unexpected given its blown-out appearance.
© Long et al 2025Modeling results showing the supernova age versus ejecta mass for the northern (red) and southern (blue) regions compared to the age estimate from an optical study of N132D. For the explosion energies and estimated ejecta masses between 2 and 6 solar masses, the models are consistent for all three.
In addition to determining the velocity of the forward shock, the authors model the evolution of the supernova remnant to estimate the initial conditions of N132D including progenitor star mass, explosion energy, and ejecta mass. Their results agree with other studies that suggested N132D originated from a roughly 15-solar-mass star that exploded 2,500 years ago into a low-density cavity.
This study showcases the unique ability of Chandra's high-resolution instruments to carefully measure the evolution of supernova remnants, and further studies will continue to determine the detailed characteristics of supernova remnants across the Local Group.
Citation
"Chandra Large Project Observations of the Supernova Remnant N132D: Measuring the Expansion of the Forward Shock," Xi Long et al 2025 ApJ 993 136. doi:10.3847/1538-4357/ae07c7
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