Dips in the sub-TeV gamma-ray light curves from central parts of galaxies due to transiting luminous stars

Abstract

The GeV-TeV γ-ray emission is observed from the direction of the source Sgr A^*, which is identified with the SMBH in the centre of our Galaxy. According to some models this γ-ray emission might originate in the very compact, central region identified with the direct surrounding of the SMBH. Sgr A^* is surrounded by a massive nuclear star cluster. Occasionally these stars might pass close to the line of sight of the observer, resulting in partial absorption of the sub-TeV γ-ray emission. We investigate the conditions at which an absorption feature appears in the γ-ray light curves from the Galactic Centre or nuclei of other galaxies containing SMBHs. The detection of such features would allow to obtain constraints on the emission site of γ rays in active galaxies. We calculate the optical depths for γ rays in the radiation of individual massive stars, or from the whole population of stars for different parameters of the star cluster. We show that the observer with a line of sight close to the orbital plane of the star can register a γ-ray absorption dip lasting from a fraction of a day up to a few tens of days. The combined effect of the bulk absorption on the whole population of stars instead can produce a flickering of the observed emission of a red-noise type in the power spectrum of the emission. Predicted absorption features in the sub-TeV γ-ray light curves from galaxies with SMBHs should be easily detectable by the Large-Sized Telescopes of the Cherenkov Telescope Array Observatory. The discovery of such absorption features provides a unique indication that the γ-ray production is occurring in a compact region, close to the horizon of the SMBH.

8 pages, 5 figures, accepted by Astronomy & Astrophysics, For the purpose of Open Access, the author has applied a CC-BY public copyright licence to any Author Accepted Manuscript (AAM) version arising from this submission

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