Absorption effects in the blazar's gamma-ray spectra due to luminous stars crossing the jet

TL;DR

This paper investigates how luminous stars crossing the jet in active galaxies can cause transient absorption features in gamma-ray spectra, with predictions applicable to observations by future Cherenkov telescopes.

Contribution

It introduces a model for time-dependent gamma-ray spectral modifications caused by stellar encounters near supermassive black holes in active galaxies.

Findings

Gamma-ray spectra steepen at tens of GeV during stellar encounters.

Spectral hardening occurs in the hundreds of GeV range.

Future Cherenkov Telescope Array can detect these transient features.

Abstract

Gamma-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. Gamma rays have to pass through the surrounding massive stellar cluster 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 gamma rays in multi-GeV to TeV energy range. We consider the effect of such stellar encounters on the gamma-ray spectrum produced within the massive stellar cluster surrounding a central SMBH. We predict characteristic, time-dependent effects on the gamma-ray spectra due to the encounter with the single luminous star and also stellar binary system. We conclude that during the encounter, the gamma-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 gamma-ray spectra, observed from blazars and radio galaxies, lays within the sensitivity of the future Cherenkov Telescope Array.