The Energy Budget of the Isotropic Diffuse Gamma-Ray Background: How Much Light the Unresolved Sky Contains

Integrating the published Fermi Large Area Telescope spectrum of the isotropic diffuse gamma-ray background over 0.1 to 820 GeV gives a self-contained energy budget for the unresolved sky. The calculation finds normalisation I0 ph cm2 s sr GeV = 9.517e-05; photon flux above 100MeV = 7.200e-06; energy flux erg cm2 s sr = 4.264e-09; energy density eV cm3 = 1.116e-06. The mean photon energy, the decade that carries most of the energy flux, and the ratio of the background energy density to that of the cosmic microwave background are all derived from the same integration, so the budget can be checked as a whole rather than quoted piecemeal. The study is simulated and computed from published values, not measured.