Persistent $\gamma$-ray emission of the blazar PKS 1510-089 in its low state: Fermi-LAT data analysis and theoretical modelling

Blazars may accelerate protons and/or nuclei as well as electrons. The hadronic component of accelerated particles in blazars may constitute the bulk of their high-energy budget; nevertheless, this component is elusive due to a high value of the energy threshold of proton interaction with photon fields inside the source. However, broad line regions (BLRs) of some flat spectrum radio quasars (FSRQs) may contain a sufficient amount of matter to render primary protons "visible" in $\gamma$ rays via hadronuclear interactions. In the present paper we study the persistent $\gamma$-ray emission of the FSRQ PKS 1510-089 in its low state utilizing the publicly-available Fermi-LAT data, as well as using the spectrum measured with the MAGIC imaging atmospheric Cherenkov telescopes. We find an indication for an excess of $\gamma$ rays at the energy range $\gtrsim$10 GeV with respect to a simple baseline log-parabolic intrinsic spectral model. The statistical significance of the excess is $\approx$5.5$\sigma$, but it drops to $\approx$2.7$\sigma$ after the account of the systematic uncertainty of the MAGIC measurements. This excess could be explained in a scenario invoking hadronuclear interactions of primary protons on the BLR material with the subsequent development of electromagnetic cascades in photon fields. We present a Monte Carlo calculation of the spectrum of this cascade component, taking as input the BLR photon field spectrum calculated with the Cloudy code. To our knowledge, this is the first calculation of electromagnetic cascade spectrum inside a blazar based on a direct calculation of the photon field spectrum with a spectral synthesis code.