Magnetospheric Gamma-Ray Emission in Active Galactic Nuclei

Abstract

The rapidly variable, very high-energy (VHE) gamma-ray emission from Active Galactic Nuclei (AGN) has been frequently associated with non-thermal processes occurring in the magnetospheres of their supermassive black holes. The present work aims to explore the adequacy of different gap-type (unscreened electric field) models to account for the observed characteristics. Based on a phenomenological description of the gap potential, we estimate the maximum extractable gap power $L_{gap}$ for different magnetospheric set-ups, and study its dependence on the accretion state of the source. $L_{gap}$ is found to be in general proportional to the Blandford-Znajek jet power $L_{BZ}$ and a sensitive function of gap size $h$, i.e. $L_{gap}\sim L_{BZ}(h/r_g{)}^{\beta }$, where the power index $\beta \ge 1$ is dependent on the respective gap-setup. The transparency of the black hole vicinity to VHE photons generally requires a radiatively inefficient accretion environment and thereby imposes constraints on possible accretion rates, and correspondingly on $L_{BZ}$. Similarly, rapid variability, if observed, may allow to constrain the gap size $h\sim c\Delta t$. Combining these constraints, we provide a general classification to assess the likelihood that the VHE gamma-ray emission observed from an AGN can be attributed to a magnetospheric origin. When applied to prominent candidate sources these considerations suggest that the variable (day-scale) VHE activity seen in the radio galaxy M87 could be compatible with a magnetospheric origin, while such an origin appears less likely for the (minute-scale) VHE activity in IC310.

13 pages, 2 figures, ApJ in press

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• Convex X-ray Spectra of PKS 2155-304 and Constraints on the Minimum Electron Energy
• Constraining cosmic-ray acceleration in the magnetospheric gaps of Sgr A*