Prospects for future very high-energy gamma-ray sky survey: Impact of secondary gamma rays

Very high-energy gamma-ray measurements of distant blazars can be well explained by secondary gamma rays emitted by cascades induced by ultra-high-energy cosmic rays. The secondary gamma rays will enable one to detect a large number of blazars with future ground based gamma-ray telescopes such as Cherenkov Telescope Array (CTA). We show that the secondary emission process will allow CTA to detect 100, 130, 150, 87, and 8 blazars above 30 GeV, 100 GeV, 300 GeV, 1 TeV, and 10 TeV, respectively, up to z ∼ 8 assuming the intergalactic magnetic field (IGMF) strength B = 10 - 17 G and an unbiased all sky survey with 0.5 h exposure at each field of view, where total observing time is ∼ 540 h. These numbers will be 79, 96, 110, 63, and 6 up to z ∼ 5 in the case of B = 10 - 15 G. This large statistics of sources will be a clear evidence of the secondary gamma-ray scenarios and a new key to studying the IGMF statistically. We also find that a wider and shallower survey is favored to detect more and higher redshift sources even if we take into account secondary gamma rays.