2019ApJ...884...91P

Starburst galaxies are huge reservoirs of cosmic rays (CRs) and these CRs convert a significant fraction of their energy into gamma-rays by colliding with the interstellar medium (ISM). The produced GeV gamma-ray emission is temporally stable, and the flux is found to correlate well with indicators of star formation rates, such as the total infrared (IR) luminosity L_IR and monochromatic radio continuum luminosity at 1.4 GHz L_1.4GHz, i.e., following L_γ - L_IR and L_γ-L_1.4GHz relations. Recently, gamma-ray excesses have been reported to be spatially coincident with two starburst galaxies, NGC 3424 and UGC 11041, in the fourth Fermi Large Area Telescope (LAT) source catalog (4FGL). Different from other starburst galaxies detected by Fermi-LAT, we find that the gamma-ray emission associated with NGC 3424 and UGC 11041 shows significant flux variability. With relatively weak infrared and radio emission, NGC 3424 and UGC 11041 appear as outliers of the L_γ - L_IR and L_γ-L_1.4GHz relations of starburst galaxies. These results suggest that NGC 3424 and UGC 11041 may harbor obscured active galactic nuclei (AGNs) and the AGN activities provide the dominant contribution to the gamma-ray emission as compared to that provided by the starburst activities.