Constraining the gamma-ray efficiency of LINER outflows with Fermi-LAT and MEGARA
Low-Ionization Nuclear Emission-line Regions (LINERs) commonly host ionized gas outflows, but their role as high-energy particle accelerators remains debated, particularly following the very-high-energy $γ$-ray detection of NGC 4278, which implied extreme radiative efficiencies. We empirically determine the $γ$-ray radiative efficiency of a local sample of LINERs to test whether their high-energy emission can be powered by extended ionized outflows or requires compact nuclear jets. We combine spatially resolved optical integral-field kinematics from the Multi-Espectrógrafo en GTC de Alta Resolución para Astronomía at the Gran Telescopio de Canarias, yielding ionized-outflow kinetic powers, with 17 years of Fermi-Large Area Telescope observations to derive 0.05--500 GeV luminosities or 95% confidence upper limits. We place the sample in an optical/$γ$-ray diagram alongside archetypal starbursts and radio galaxies. We present the first empirical upper limits on the $γ$-ray radiative efficiency of LINER outflows as a population. No LINER is formally detected ($TS \geq 16$). The strongest constraint is obtained for the radio-loud LINER NGC 1052, with $η< 41%$. For the remaining sources, the Fermi-LAT limits generally lie well above the outflow kinetic powers ($η\gg 100%$), while three sources show marginal hints of emission ($9 < TS < 16$). We conclude that extended ionized outflows in LINERs are highly inefficient high-energy particle accelerators, analogous to starburst superwinds. These constraints disfavor the outflows as the sole origin of extreme $γ$-ray efficiencies and favor compact nuclear jets for the most efficient $γ$-ray-emitting LINERs.