Gamma-ray emission toward supergiant shells in the Large Magellanic Cloud

Abstract

The Large Magellanic Cloud (LMC), a satellite galaxy of our Milky Way, is considered to be a very good target for investigations of diffuse gamma-ray emission. Observations have shown that diffuse emissions are related to the large- and small-scale structures of the LMC. Particularly, it has been suggested that some of the small-scale bright diffuse emissions coincide with supergiant shells (SGSs) previously identified based on H$\alpha$ images. We analyzed the gamma-ray emission from the LMC using more than 15 years of observations with the \textit{Fermi} Large Area Telescope in the energy range of 300MeV-300GeV. A dedicated spatial and spectral analysis was made for diffuse emission components of the LMC. Particularly, we evaluated the emissions from SGSs in detail using phenomenological approaches, and we established their corresponding spatial templates for gamma-ray emission. In the analysis of small-scale extended emissions, we detected seven H$\alpha$-selected SGSs at a significance of more than $5\sigma$. The emission structure nested inside the SGSs appears to be a non-Gaussian feature and is very likely due to a modulated cosmic-ray distribution by an ensemble of supernova remnant shocks. The spectra of the gamma-ray emission for most SGS regions favor a hadronic origin. For the large-scale diffuse emission from the LMC disk, our results suggest an interplay of hadron- and lepton-induced emission. The cosmic-ray proton energy density derived for bright SGSs is 3-10 times higher than that in the LMC disk. SGS$2$ was detected to be the most significant emission region among all SGSs. Based on the gamma-ray excess emission we detected in H$\alpha$-selected SGSs in the LMC, we estimate that their contribution to the total diffuse gamma-ray emission in the LMC is at a non-negligible 41\% level.

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