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Gamma-ray observations of active galaxies have important consequences for theories of the activity in their nuclei. The observations of Cen A, 3C 273, and NGC 4151 are examined under the assumption that Penrose collision processes in the ergospheres of massive black holes power their nuclei. The observed sharp break in the MeV region of the NGC 4151 spectrum cannot be due to the γ-γ pair production process. We attribute this break to the Penrose Compton scattering (PCS), in which γ-rays escape from the ergosphere as a result of Penrose processes involving electrons and lower energy X-ray photons in the ergosphere of the black hole. The absence of an MeV break in the spectra of Cen A and 3C 273 argues in favor of the Penrose pair production (PPP), in which high-energy pairs (a few GeV in energy) escape as result of Penrose processes involving protons and γ-rays that are present in any hot, optically thin, vertically extended accretion disk. An intrinsic break in the GeV region is predicted for both Cen A and 3C 273 as well as any other PPP powered nucleus. The mass of the black hole, the accretion rate, the efficiency of accretion, etc., are obtained self-consistently under the assumption that γ-γ scattering is unimportant below ~10-100 MeV. This assumption is very reasonably based on the observations. Central black hole masses of tens of millions solar masses for NGC 4151 and Cen A, and tens of billions solar masses for 3C 273, are obtained. Even though fewer Penrose produced pairs are emitted along the rotation axis of the hole, they suffer smaller energy losses in that direction, and therefore PPP has built into it the possibility of contributing to the explanation of the lobes of radio galaxies. The effect of "aging" of the active nucleus may account for some of the differences among different types of Seyferts (and presumably QSOs) since objects which are spinning close to the canonical value of af M = 0.998 would have Penrose processes but not those which have slowed down their spin. If PPP is important for QSOs and radio galaxies and some Seyferts, we expect powerful radio objects to be also powerful γ-ray objects. Nuclei in which the black hole is spinning slowly would still emit visible light, UV, and X-rays as result of accretion without Penrose processes but would be weak in radio or high-energy γ-rays. Future γ-ray observations should provide clues as to whether this scenario is correct. Besides spectral information at γ-ray frequencies, possible variability at γ-ray frequencies should be searched for.


This article was originally published in Astrophysical Journal, volume 236, in 1980. DOI: 10.1086/157723

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IOP Publishing



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