Prospects for γ-ray observations of narrow-line Seyfert 1 galaxies with the Cherenkov Telescope Array. II. γ-γ absorption in the broad-line region radiation fields

Abstract

Gamma-ray emitting narrow-line Seyfert 1 (γ-NLS1) galaxies possibly harbour relatively low-mass black holes (106-108 M⊙) accreting close to the Eddington limit, and share many characteristics with their sibling sources, flat-spectrum radio quasars. Although they have been detected in the MeV-GeV band with Fermi-LAT, they have never been seen in the very high energy band with current imaging atmospheric Cherenkov telescopes (IACTs). Thus, they are key targets for the next-generation IACT, the Cherenkov Telescope Array (CTA). In a previous work we selected, by means of extensive simulations, the best candidates for a prospective CTA detection (SBS 0846+513, PMN J0948+0022, and PKS 1502+036) taking into account the effects of both the intrinsic absorption (approximated with a cut-off at 30 GeV), and the extragalactic background light on the propagation of γ-rays. In this work, we simulate the spectra of these three sources by adopting more realistic broad-line region (BLR) absorption models. In particular, we consider the detailed treatment of γ-γ absorption in the radiation fields of the BLR as a function of the location of the γ-ray emission region with parameters inferred from observational constraints. We find that, due to the energy range extent and its sensitivity, CTA is particularly well suited to locate the γ-ray emitting region in γ-NLS1. In particular CTA will be able not only to distinguish whether the γ-ray emitting region is located inside or outside the BLR, but also where inside the BLR it may be