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

TL;DR

This paper explores how transiting luminous stars near supermassive black holes can cause detectable dips in sub-TeV gamma-ray light curves, offering insights into the gamma-ray emission regions in galactic centers.

Contribution

It models gamma-ray absorption caused by stars near SMBHs and predicts observable features for upcoming Cherenkov Telescope Array observations.

Findings

Absorption dips last from hours to days depending on star proximity.

Flickering in gamma-ray emission can result from collective stellar effects.

Features are detectable by future gamma-ray observatories.

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.