Fermi Large Area Telescope Observations of the Monoceros Loop Supernova Remnant

We present an analysis of the gamma-ray measurements by the Large Area Telescope onboard the $\mathit{\text{Fermi}}\mathit{\text{Gamma-ray}}\mathit{\text{Space}}\mathit{\text{Telescope}}$ in the region of the supernova remnant~(SNR) Monoceros Loop~(G205.5$+$0.5). The brightest gamma-ray peak is spatially correlated with the Rosette Nebula, which is a molecular cloud complex adjacent to the southeast edge of the SNR. After subtraction of this emission by spatial modeling, the gamma-ray emission from the SNR emerges, which is extended and fit by a Gaussian spatial template. The gamma-ray spectra are significantly better reproduced by a curved shape than a simple power law. The luminosities between 0.2–300~GeV are $\sim$~$4 \times 10^{34}$ergs$^{-1}$fortheSNRand$\sim$$3 \times 10^{34}$ergs$^{-1}$ for the Rosette Nebula, respectively. We argue that the gamma rays likely originate from the interactions of particles accelerated in the SNR. The decay of neutral pions produced in nucleon-nucleon interactions of accelerated hadrons with interstellar gas provides a reasonable explanation for the gamma-ray emission of both the Rosette Nebula and the Monoceros SNR.