Lower Bound on the Cosmic TeV Gamma-ray Background Radiation

Understand

The Fermi gamma-ray space telescope has revolutionized our understanding of the cosmic gamma-ray background radiation in the GeV band.

• However, investigation on the cosmic TeV gamma-ray background radiation still remains sparse.
• Here, we report the lower bound on the cosmic TeV gamma-ray background spectrum placed by the cumulative flux of individual detected extragalactic TeV sources including blazars, radio galaxies, and starburst galaxies.
• The current limit on the cosmic TeV gamma-ray background above 0.1 TeV is obtained as $2.8\times10^{-8} (E/100~{\rm GeV})^{-0.55} \exp(-E/2100~{\rm GeV})~{\rm [GeV/cm^2/s/sr]} < E^2dN/dE < 1.1\times10^{-7} (E/100~{\rm GeV})^{-0.49}~{\rm [GeV/cm^2/s/sr]}$, where the upper bound is set by requirement that the cascade flux from the cosmic TeV gamma-ray background radiation can not exceed the measured cosmic GeV gamma-ray background spectrum (Inoue & Ioka, 2012).