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

Abstract

The GeV-TeV {\gamma}-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 {\gamma}-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 {\gamma}-ray emission. We investigate the conditions at which an absorption feature appears in the {\gamma}-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 {\gamma} rays in active galaxies. We calculate the optical depths for {\gamma} 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 {\gamma}-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 {\gamma}-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 {\gamma}-ray production is occurring in a compact region, close to the horizon of the SMBH.