Series: Physics Colloquium
Blazar astronomy is rapidly progressing thanks in large part to the successes of the Fermi Gamma-ray Space Telescope and the ground-based gamma-ray telescopes. More than 1000 active galaxies have been detected at GeV energies, and nearly 50 at Very-High Energies (VHE, > 100 GeV). We can now explore multiwavelength and multi-messenger connections in unprecedented detail, and derive the astroparticle implications of those results. In this presentation, leptonic and hadronic spectral modeling of blazars is reviewed with the intent of identifying ultra-high energy cosmic rays (UHECRs) in the spectral energy distributions of these objects. We consider a number of unusual results that could be explained by UHECRs in blazars:
(1) distinct spectral components revealed by deabsorption of blazar VHE spectra;
(2) flattening at moderate redshift in the Stecker-Scully relation showing the GeV - TeV spectral index difference versus redshift;
(3) conflicting results for the location of the gamma-ray emission site in blazars;
(4) the unusually short variability times of luminous blazars.
The arguments for and against radio galaxies and blazars being the sources of the UHECRs are reviewed, and predictions for UHECR composition is made if BL Lac objects accelerate most of the UHECRs. Unusual effects of UHECR acceleration in blazars is illustrated by the strange case of 4C +21.35. We also discuss effects of hypothetical axions, a dark matter candidate, in the interpretation of unusual blazar behavior, and a recent Fermi-LAT search for axions in occultations of bright AGNs by the Sun.