2015A&A...584A..20W

We check how the Fermi/LAT data constrain the physics of hot accretion flows that are most likely present in low-luminosity AGNs. Using a precise model of emission from hot flows, we studied the flow γ-ray emission resulting from proton-proton interactions. We explored the dependence of the γ-ray luminosity on the accretion rate, the black hole spin, the magnetic field strength, the electron heating efficiency, and the particle distribution. Then, we compared the hadronic γ-ray luminosities predicted by the model for several nearby Seyfert 1 galaxies with the results of our analysis of 6.4 years of Fermi/LAT observations of these AGNs. In agreement with previous studies, we find a significant γ-ray detection in NGC 6814. We were only able to derive upper limits for the remaining objects, although we report marginally significant (∼3σ) signals at the positions of NGC 4151 and NGC 4258. The derived upper limits for the flux above 1GeV allow us to constrain the proton acceleration efficiency in flows with heating of electrons dominated by Coulomb interactions, which case is favored by the X-ray spectral properties. In these flows, at most ∼10% of the accretion power can be used for a relativistic acceleration of protons. Upper limits for the flux below 1GeV can constrain the magnetic field strength and black hole spin value; we find these constraints for NGC 7213 and NGC 4151. We also note that the spectral component above ∼4GeV previously found in the Fermi/LAT data of Centaurus A may be due to hadronic emission from a flow within the above constraint. We rule out this origin of the γ-ray emission for NGC 6814. For models with a strong magnetohydrodynamic heating of electrons, the hadronic γ-ray fluxes are below the Fermi/LAT sensitivity even for the closest AGNs. In these models, nonthermal Compton radiation may dominate in the γ-ray range if electrons are efficiently accelerated and the acceleration index is hard; for the index ≃2, the LAT upper limits constrain the fraction of accretion power used for such an acceleration to at most ∼5%. Finally, we note that the three Seyfert 2 galaxies with high starburst activity NGC 4595, NGC 1068, and Circinus show an interesting correlation of their γ-ray luminosities with properties of their active nuclei, and we discuss this in the context of the hot flow model.