The Origin of High-energy Emission in the Young Radio Source PKS 1718–649

Abstract

We present a model for the broadband radio-to-γ-ray spectral energy distribution of the compact radio source PKS 1718 − 649. Because of its young age (≃100 yr) and proximity (z = 0.014), PKS 1718 − 649 offers a unique opportunity to study the nuclear conditions and the jet/host galaxy feedback process at the time of the initial radio jet expansion. PKS 1718 − 649 is one of a handful of young radio jets with γ-ray emission that has been confirmed by the Fermi/LAT detector. We show that in PKS1718−649 this γ-ray emission can be successfully explained by Inverse Compton scattering of the UV photons, presumably from an accretion flow, off nonthermal electrons in the expanding radio lobes. The origin of the X-ray emission in PKS 1718 − 649 is more elusive. While Inverse Compton scattering of the IR photons emitted by a cold gas in the vicinity of the expanding radio lobes contributes significantly to the X-ray luminosity, the data require an additional source of X-rays, e.g., a weak X-ray corona or a radiatively inefficient accretion flow, as expected from a LINER-type nucleus, such as that of PKS 1718 − 649. We find that the jet in PKS 1718 − 649 has low power, L j ≃ 2 × 1042 erg s−1, and expands in an environment with density n 0 ≃ 3–20 cm−3. The inferred mass accretion rate and gas mass reservoir within 50–100 pc of the galactic center are consistent with estimates from the literature obtained by tracing molecular gas in the innermost region of the host galaxy with SINFONI and the Atacama Large Millimeter/submillimeter Array.