SIMBAD references

Low-luminosity active galactic nuclei (LLAGNs) possess the characteristic features of more luminous active galactic nuclei (AGNs) but exhibit a much lower nuclear Hα luminosity (L_Hα< 10⁴⁰ erg/s) than their more luminous counterparts. M87 (NGC 4486) and Centaurus A (NGC 5128, Cen A) are well studied nearby LLAGNs. As an additional feature they show γ radiation up to TeV (10¹² eV) energies, but the origin of this radiation has not been resolved. The coincident observation of a radio and TeV flare in M87 suggests that the TeV radiation is produced within around 50-100 gravitational radii of the central supermassive black hole, depending on the assumed value of the mass of the black hole. Strong radiation fields can be produced in the central region of an (LL)AGN, e.g., by the accretion flow around the black hole, the jet plasma, or stars closely orbiting the black hole. These radiation fields can lead to the absorption of emitted TeV photons, and, in fact, high optical depths of such fields can make TeV detection from inner regions impossible. In this paper, we consider the accretion flow around the black hole as the most prominent source for such a radiation field and we calculate accordingly the probability for absorption of TeV photons produced near the black holes in M87 and Cen A assuming a low-luminosity Shakura-Sunyaev disk (SSD). We find that the results are very different between the two LLAGNs. While the inner region of M87 is transparent for TeV radiation up to ∼20 TeV within the allowed parameter range, the optical depth in Cen A is {Gt}1, leading to an absorption of TeV photons that might be produced near the central black hole. These results imply either that the TeV γ production sites and processes are different for both sources or that LLAGN black holes do not accrete (at least only) in the form of a low-luminosity SSD.