Gamma-ray activity of Seyfert galaxies and constraints on hot accretion flows
Abstract
Aims: We check how the Fermi/LAT data constrain the physics of hot accretion flows that are most likely present in low-luminosity AGNs.
Methods: 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.
Results: 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 1 GeV 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 1 GeV 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 ~4 GeV 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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- December 2015
- DOI:
- 10.1051/0004-6361/201526621
- arXiv:
- arXiv:1505.07608
- Bibcode:
- 2015A&A...584A..20W
- Keywords:
-
- black hole physics;
- accretion;
- accretion disks;
- galaxies: Seyfert;
- gamma rays: galaxies;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- A&