The characteristics of blazars that are sources of very-high-energy gamma rays

We show that the most probable extragalactic sources of very-high-energy gamma rays are HBL blazars whose peak frequencies are in the X-ray. The detection of very-high-energy gamma rays from the blazar 3C66A, which has a redshift of z=0.44, suggests that the density of the intergalactic infrared background at wavelengths >0.6 µm is lower than estimates published in the literature.