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

TL;DR

Future gamma-ray surveys with CTA could detect hundreds of distant blazars via secondary gamma rays, providing insights into cosmic ray cascades and intergalactic magnetic fields, especially with optimized survey strategies.

Contribution

This study predicts the number of blazars detectable by CTA through secondary gamma rays, highlighting the importance of survey design and magnetic field assumptions for future observations.

Findings

CTA can detect up to 150 blazars above 30 GeV at z~8 with secondary gamma rays.

Survey strategy impacts detection rates, with wider, shallower surveys being more effective.

Detection of these sources will test secondary gamma-ray models and probe intergalactic magnetic fields.

Abstract

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 $z\sim8$ assuming the intergalactic magnetic field (IGMF) strength $B=10^{-17}$ G and an unbiased all sky survey with 0.5 hr exposure at each Field of View, where total observing time is $\sim540$ hr. These numbers will be 79, 96, 110, 63, and 6 up to $z\sim5$ 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.