Probing the γ-Ray Emission Origin of Two Star-forming Galaxies NGC 2403 and NGC 3424 with the Fermi-LAT

Star-forming galaxies (SFGs) are a subclass of γ -ray emitters, and a correlation between their γ -ray luminosity ( L _γ ) and the total infrared (IR) luminosity ( L _IR ) has been established based on the Fermi Large Area Telescope (LAT) data. NGC 2403 and NGC 3424 have been reported as outliers in the L _γ – L _IR correlation with light curves showing significant variability, which contrasts with the temporally stable γ -ray emission in other SFGs, originating primarily from cosmic rays interacting with interstellar medium. In this study, we reanalyze the γ -ray emission in the directions of NGC 2403 and NGC 3424 using more than 16.5 yr Fermi-LAT data. NGC 3424 is found to be spatially coincident with the detected γ -ray source, while NGC 2403 is significantly offset from the nearest γ -ray source, suggesting an implausible association. We confirm the previously reported variability of both γ -ray sources and the significant deviation from the L _γ – L _IR correlation when assuming an association of both γ -ray sources with the two galaxies. Our findings lend further support to the interpretation that their γ -ray emission is driven primarily by alternative radiative processes—rather than by star formation activity—such as the ejecta of the Type IIP supernova SN 2004dj in NGC 2403 interacting with a surrounding high-density shell and an obscured active galactic nucleus in NGC 3424.