LMC N132D: A mature supernova remnant with a power-law gamma-ray spectrum extending beyond 8 TeV

TL;DR

This study presents the detection and detailed spectral analysis of the supernova remnant N132D in the Large Magellanic Cloud, revealing a power-law gamma-ray spectrum extending beyond 8 TeV and supporting a hadronic emission origin.

Contribution

The paper provides the first combined GeV-TeV gamma-ray spectrum of N132D, establishing its high-energy extension and setting a lower limit on its cutoff energy, with implications for cosmic ray acceleration models.

Findings

N132D is detected at very high energies with 5.7 sigma significance.

The gamma-ray spectrum extends beyond 8 TeV without a cutoff.

The emission is likely hadronic in origin, influenced by nearby molecular cloud interactions.

Abstract

We analyzed 252 hours of High Energy Stereoscopic System (H.E.S.S.) observations towards the supernova remnant (SNR) LMC N132D that were accumulated between December 2004 and March 2016 during a deep survey of the Large Magellanic Cloud, adding 104 hours of observations to the previously published data set to ensure a > 5 sigma detection. To broaden the gamma-ray spectral coverage required for modeling the spectral energy distribution, an analysis of Fermi-LAT Pass 8 data was also included. We unambiguously detect N132D at very high energies (VHE) with a significance of 5.7 sigma. We report the results of a detailed analysis of its spectrum and localization based on the extended H.E.S.S. data set. The joint analysis of the extended H.E.S.S and Fermi-LAT data results in a spectral energy distribution in the energy range from 1.7 GeV to 14.8 TeV, which suggests a high luminosity of N132D at GeV and TeV energies. We set a lower limit on a gamma-ray cutoff energy of 8 TeV with a confidence level of 95%. The new gamma-ray spectrum as well as multiwavelength observations of N132D when compared to physical models suggests a hadronic origin of the VHE gamma-ray emission. SNR N132D is a VHE gamma-ray source that shows a spectrum extending to the VHE domain without a spectral cutoff at a few TeV, unlike the younger oxygen-rich SNR Cassiopeia A. The gamma-ray properties of N132D may be affected by an interaction with a nearby molecular cloud that partially lies inside the 95% confidence region of the source position. [Abridged]