Evidence of AGN activity in the gamma-ray emission from two starburst
  galaxies
  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 $L_{\rm IR}$
and monochromatic radio continuum luminosity at 1.4 GHz $L_{\rm 1.4\ GHz}$,
i.e., following $L_{\gamma}-L_{\rm IR}$ and $L_{\gamma}-L_{\rm 1.4\ GHz}$
relations. Recently, gamma-ray excesses are reported to be spatially coincident
with two starburst galaxies NGC 3424 and UGC 11041 in the fourth \textsl{Fermi}
Large Area Telescope (LAT) source catalog (4FGL). Different from other
starburst galaxies detected by \textsl{Fermi}-LAT, we find that the gamma-ray
emission associated with NGC 3424 and UGC 11041 show significant flux
variability. With relatively weak infrared and radio emission, NGC 3424 and UGC
11041 appear as outliers of the $L_{\gamma}-L_{\rm IR}$ and $L_{\gamma}-L_{\rm
1.4\ GHz}$ 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.
