Probing the $\gamma$-ray Emission Origin of Two Star-forming Galaxies NGC 2403 and NGC 3424 with the Fermi-LAT

Abstract

Star-forming galaxies (SFGs) are a subclass of $\gamma$-ray emitters and a correlation between their $\gamma$-ray luminosity ($L_{\rm \gamma}$) and the total infrared (IR) luminosity ($L_{\rm IR}$) has been established based on the Fermi Large Area Telescope (LAT) data. NGC 2403 and NGC 3424 have been reported as outliers in the $L_{\rm \gamma}$-$L_{\rm IR}$ correlation with light curves showing significant variability, which contrasts with the temporally stable $\gamma$-ray emission in other SFGs, originating primarily from cosmic rays interacting with interstellar medium. In this study, we reanalyze the $\gamma$-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 $\gamma$-ray source, while NGC 2403 is significantly offset from the nearest $\gamma$-ray source, suggesting an implausible association. We confirm the previously reported variability of both $\gamma$-ray sources and the significant deviation from the $L_{\rm \gamma}$-$L_{\rm IR}$ correlation when assuming an association of both $\gamma$-ray sources with the two galaxies. Our findings lend further support to the interpretation that their $\gamma$-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.