Internal gamma gamma-opacity in Active Galactic Nuclei and the consequences for the TeV observations of M87 and Cen A

Abstract

Low Luminosity Active Galactic Nuclei (LLAGNs) possess the characteristic features of more luminous Active Galactic Nuclei (AGNs) but exhibit a much lower nuclear Halpha luminosity than their more luminous counterparts. M87 (NGC 4486) and Centaurus A (NGC 5128, CenA) are well-studied nearby LLAGNs. As an additional feature they show gamma-radiation up to TeV (10^{12}eV) energies, but the origin of this radiation is not 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 accordingly calculate the probability for absorption of TeV photons produced near the black holes in M87 and CenA assuming a low luminosity Shakura-Sunyaev Disk (SSD). We find that the results are very different for between the two LLAGNs. While the inner region of M87 is transparent for TeV radiation up to 15TeV, the optical depth in CenA is >> 1, leading to an absorption of TeV photons that might be produced near the central black hole. These results imply either that the TeV gamma production sites and processes are different for both sources, or that LLAGN black holes do not accrete (at least only) in form of a low luminosity SSD.

accepted for publication in ApJ

References in corpus (14)

• Fast variability of TeV gamma-rays from the radio galaxy M 87
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Cited by in corpus (11)

• The 2010 very high energy gamma-ray flare & 10 years of multi-wavelength observations of M 87
• 230 GHz VLBI observations of M87: event-horizon-scale structure at the enhanced very-high-energy $\gamma$-ray state in 2012
• A strong radio brightening at the jet base of M87 during the elevated very-high-energy gamma-ray state in 2012
• VLBI Observations of the Jet in M87 During the Very-High-Energy Gamma-ray Flare in April 2010
• Ruling out thermal dark matter with a black hole induced spiky profile in the M87 galaxy
• Broadband Multi-wavelength Properties of M87 during the 2018 EHT Campaign including a Very High Energy Flaring Episode
• Probing the central black hole in M87 with gamma-rays
• Curvature in the very-high energy gamma-ray spectrum of M87
• Detection of very high-energy gamma-ray emission from the radio galaxy M87 with LHAASO
• Limits on the source properties of FR-I galaxies from high-energy neutrino and gamma observations
• Broadband Modeling of Low Luminosity Active Galactic Nuclei Detected in Gamma Rays