Optical depth of photopion production inside and outside the source emission region for TXS 0506+056. The target photons for pγ interactions inside the jet are leptonic synchrotron and SSC radiation, as shown in Fig. 3 of Gao et al. (2019) for their favored hybrid model. The parameter values given in their supplementary Table 1 for the flare state are used to calculate the photon flux in the comoving jet frame.