The spectral evolution of gamma‐ray pulsars

The six known gamma‐ray pulsars all have varying spectral characteristics between optical and TeV gamma‐rays. These spectra were used to study the conversion efficiency η of spindown power to γ‐ray luminosity and mean energy as a function of age τ. The possibility of reaching different conclusions, when using total spectra in contrast with spectra for E≳100 MeV, is investigated. The data are consistent with the original suggestion of η∝τ, as well as with a more detailed model stating η∝P1.3τ1.8. The uncertainties involved does not allow us to discriminate between these two models. The mean energy 〈E〉 increases rapidly with age for 103<τ<104 years, with 〈E〉∼400 eV for the Crab, but saturates at 〈E〉∼200 MeV when τ≫104 years. These results suggest that the emission mechanism changes as the pulsar ages beyond 104 years.