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

Abstract

Context. The GeV-TeV γ-ray emission was observed from the direction of the source Sgr A^{⋆, which is identified with the supermassive black hole (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, composed of the S stars, also including OB-type stars. 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.Aims. We investigated 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 us to obtain constraints on the emission site of γ rays in active galaxies.Mehods. We calculated 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.Results. 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.Conclusions. 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.}