On the hadronuclear origin of the TeV emission of Ap Librae

We investigate the origin of the very-high-energy (VHE; ≥ 100 GeV) γ-ray emission from the low-synchrotron-peaked BL Lac object Ap Librae within the framework of a one-zone hadronuclear (pp) interaction model. Conventional leptonic and hadronic scenarios (including photohadronic processes and proton synchrotron emission), have encountered difficulties in reproducing the unusually broad GeV-TeV spectrum of this source without invoking extreme physical conditions. In this work, we construct a one-zone pp model that incorporates both leptonic and hadronuclear components, and apply it to fit the multiwavelength spectral energy distribution of Ap Librae. Our results show that the optical-GeV emission can be well explained by leptonic emission from primary electrons, while the TeV emission is naturally accounted for by γ-rays from neutral pion decay in pp interactions. The required total jet power is found to be ~1.8 × 10⁴⁶ erg s^-1, which corresponds to half of the Eddington luminosity of the central supermassive black hole. Compared with previous hadronic models that demand super-Eddington energetics, our approach provides a more physically reasonable explanation for the VHE spectrum of Ap Librae.