2020MNRAS.495.3463G

Within the classical emission model, where the emission region is placed within the broad line region (BLR), flat spectrum radio quasars (FSRQs) were believed not to emit photons with energies above few tens of GeV because of the absorption with the optical-UV photons from the BLR. However, photons with observed energies up to about 300 GeV have been detected for few FSRQs, whose most iconic example is PKS 1441+25 at redshift z = 0.94. The most conservative explanation for these observations is that the emission occurs at distances comparable to the size of the dusty torus. In this case, absorption of high-energy gamma-ray photons for energies above 200-300 GeV is dominated by the interaction with infrared radiation emitted by the torus. We investigate if current observational data about FSRQs in flaring state can give us information about: (i) the importance of the torus absorption and (ii) the properties of the torus i.e. its temperature and its geometry. We find that present data do not arrive at energies where the torus influence is prominent and as a result it is currently hardly possible to infer torus properties from observations. However, with dedicated simulations, we demonstrate that observations with the forthcoming Cherenkov Telescope Array (CTA) will be able to constrain the torus parameters (temperature and geometry).