Where are the Cascades from Blazar Jets? An Emerging Tension in the $\gamma$-ray sky

Abstract

Blazars are among the most powerful accelerators and are expected to produce a bright TeV $\gamma$-ray flux. However, TeV $\gamma$-rays are attenuated by interactions with intergalactic radiation before reaching Earth. These interactions produce cascades that transfer TeV power into the GeV band, powering both extended halos around bright sources and a large contribution to the isotropic $\gamma$-ray background (IGRB). Using conservative blazar models and recent IGRB measurements, we rule out scenarios where blazars effectively transfer their multi-TeV power into GeV $\gamma$-rays. Three possible solutions include: (1) strong spectral cuts on bright blazars, which are increasingly in tension with local blazar data, (2) collective plasma effects that can prevent the development of blazar cascades, the effectiveness of which is debated, (3) an increase in the $\gamma$-ray opacity from axion-like particles.