Study of Flat Spectrum Radio Quasars and BL Lacertae Objects as Sources of Diffusive Ultra High-Energy Cosmic Rays

Abstract

We examine whether Flat Spectrum Radio Quasars (FSRQs) and BL Lacertae objects (BL Lacs) can act as plausible astrophysical sources of diffuse ultra-high-energy cosmic rays (UHECRs). Using realistic luminosity-dependent density evolution (LDDE) functions derived from observed gamma-ray luminosity functions for FSRQs and BL Lacs, we calculate the redshift evolution of the cosmic ray source population through integrated luminosity functions. The diffuse UHECRs flux from these sources is modelled by propagating nuclei through extragalactic space, including energy losses from interactions with cosmic photon backgrounds. The resulting UHECRs spectra are compared with observational data from the Pierre Auger Observatory and the Telescope Array, with fluxes normalised at reference energies. We also construct illustrative full skymaps of the integral UHECRs flux from $10$ EeV to $100$ EeV using the \textsc{HEALPix} framework, assuming a uniform distribution of synthetic sources and including energy dependent magnetic diffusion. Our results show that although both FSRQs and BL Lac objects can reproduce the observed UHECR flux, the FSRQ scenario is disfavoured by anisotropy and inconsistent source density, while BL Lacs remain consistent with observations.