Abstract
ABSTRACT γ-ray emission in active galaxies is likely produced within the inner jet, or in the close vicinity of the supermassive black hole (SMBH) at sub-parsec distances. γ-rays have to pass through the surrounding massive stellar cluster in which luminous stars can accidentally appear close to the observer’s line of sight. In such a case, soft radiation of massive stars can create enough target for transient absorption of the γ-rays in multi-GeV to TeV energy range. We consider the effect of such stellar encounters on the γ-ray spectrum produced within the massive stellar cluster surrounding a central SMBH. We predict characteristic, time-dependent effects on the γ-ray spectra due to the encounter with the single luminous star and also stellar binary system. We conclude that during the encounter, the γ-ray spectrum of an active galaxy should steepen at tens of GeV and harden in the range of hundreds of GeV. As an example, we consider such effects on the spectra observed from a typical blazar, 1ES 1959+650 (in an active state) and also in the case of a radio galaxy M87 (in a low state). It is shown that observation of such transient characteristic features in the γ-ray spectra, observed from blazars and radio galaxies, lays within the sensitivity of the future Cherenkov Telescope Array.
Highlights
It is supposed that high-energy γ rays from active galaxies are produced within the inner jet or in the magnetosphere of the supermassive black hole (SMBH) due to their extremely short variability time scale
In the case of some blazars, the TeV γ-ray emission can change significantly on a time scale as short as minutes (e.g. Mrk 501, see Albert et al 2007; PKS 2155-304, see Aharonian et al 2007 or inverse Compton (IC) 310, see Aleksic et al 2014), suggesting that the emission site has to be close to the SMBH, even if a mild relativistic boosting is taken into account
We considered the effects of transition of luminous stars through the γ-ray beam produced in the direct surrounding of the SMBH, i.e. either in the SMBH magnetosphere or in the inner part of the jet
Summary
It is supposed that high-energy γ rays from active galaxies are produced within the inner jet or in the magnetosphere of the supermassive black hole (SMBH) due to their extremely short variability time scale. We investigate the details of such effects on the γ-ray spectrum in the case of a transition of either a single or a binary system of two luminous stars close to the line of sight of a distant observer.
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