LMC N132D: a mature supernova remnant with a youthful gamma-ray spectrum

TL;DR

LMC N132D is a luminous, mature supernova remnant with a powerful gamma-ray spectrum extending above 10 TeV, indicating hadronic cosmic-ray acceleration and interaction with a molecular cloud, surpassing many other known remnants in TeV luminosity.

Contribution

This study provides the first significant detection of N132D at very high energies, revealing an extended power-law gamma-ray spectrum with no cutoff below 8 TeV, and suggests a hadronic origin linked to molecular cloud interaction.

Findings

N132D detected at 5.7σ significance in VHE gamma rays.

Gamma-ray spectrum extends above 10 TeV with no cutoff below 8 TeV.

N132D's TeV luminosity exceeds that of Cas A by over an order of magnitude.

Abstract

The supernova remnant LMC N132D is a remarkably luminous gamma-ray emitter at $\sim$50 kpc with an age of $\sim$2500 years. It belongs to the small group of oxygen-rich SNRs, which includes Cassiopeia A (Cas A) and Puppis A. N132D is interacting with a nearby molecular cloud. By adding 102 hours of new observations with the High Energy Stereoscopic System (H.E.S.S.) to the previously published data with exposure time of 150 hours, we achieve the significant detection of N132D at a 5.7$\sigma$ level in the very high energy (VHE) domain. The gamma-ray spectrum is compatible with a single power law extending above 10 TeV. We set a lower limit on an exponential cutoff energy at 8 TeV with 95% CL. The multi-wavelength study supports a hadronic origin of VHE gamma-ray emission indicating the presence of sub-PeV cosmic-ray protons. The detection of N132D is remarkable since the TeV luminosity is higher than that of Cas A by more than an order of magnitude. Its luminosity is comparable to, or even exceeding the luminosity of RX J1713.7-3946 or HESS J1640-465. Moreover, the extended power-law tail in the VHE spectrum of N132D is surprising given both the exponential cutoff at 3.5 TeV in the spectrum of its 340-year-old sibling, Cassiopeia A, and the lack of TeV emission from a Fermi- LAT 2FHL source (E > 50 GeV) associated with Puppis A. We discuss a physical scenario leading to the enhancement of TeV emission via the interaction between N132D and a near molecular cloud.