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G326.3$-$1.8 (also known as MSH 15$-$56) has been detected in radio as amiddle-aged composite supernova remnant (SNR) consisting of an SNR shell and apulsar wind nebula (PWN), which has been crushed by the SNR's reverse shock.Previous $\gamma$-ray studies of SNR G326.3$-$1.8 revealed bright and extendedemission with uncertain origin. Understanding the nature of the $\gamma$-rayemission allows probing the population of high-energy particles (leptons orhadrons) but can be challenging for sources of small angular extent. With therecent $\textit{Fermi}$ Large Area Telescope data release Pass 8, weinvestigate the morphology of this SNR to disentangle the PWN from the SNRcontribution. We perform a morphological and spectral analysis from 300 MeV to300 GeV. We use the reconstructed events with the best angular resolution toseparately investigate the PWN and the SNR emissions, which is crucial toaccurately determine the spectral properties of G326.3$-$1.8 and understand itsnature. The centroid of the $\gamma$-ray emission evolves with energy and isspatially coincident with the radio PWN at high energies (E $>$ 3 GeV). Themorphological analysis reveals that a model considering two contributions fromthe SNR and the PWN reproduces the $\gamma$-ray data better than asingle-component model. The associated spectral analysis using power laws showstwo distinct spectral features, a softer spectrum for the remnant ($\Gamma$ =2.17 $\pm$ 0.06) and a harder spectrum for the PWN ($\Gamma$ = 1.79 $\pm$0.12), consistent with hadronic and leptonic origin for the SNR and the PWNrespectively. Focusing on the SNR spectrum, we use one-zone models to derivesome physical properties and, in particular, we find that the emission is bestexplained with a hadronic scenario in which the large target density isprovided by radiative shocks in HI clouds struck by the SNR.

Disentangling hadronic from leptonic emission in the composite SNR G326.3$-$1.8