Se p 19 97 Phase - Resolved Studies of the High - Energy Gamma - Ray Emission from the Crab , Geminga , and Vela Pulsars

Using the first three and a half years of observations from the Energetic Gamma Ray Telescope (EGRET) on board the Compton Gamma Ray Observatory (CGRO), phase-resolved analyses are performed on the emission from the three brightest highenergy γ-ray pulsars, Crab, Geminga, and Vela. For each pulsar, it is found that there is detectable high-energy γ-ray emission above the galactic diffuse background throughout much of the pulsar rotation cycle. A hardness ratio is introduced to characterize the evolution of the spectral index as a function of pulsar phase. While the hardest emission from the Crab and Vela pulsars comes from the bridge region between the two γ-ray peaks, the hardest emission from Geminga corresponds to the second γ-ray peak. For all three pulsars, phase-resolved spectra of the pulse profile components reveal that although there is a large variation in the spectral index over the pulsar phase interval, the high-energy spectral turnover, if any, occurs at roughly the same energy in each component. The high-energy γ-ray emission from the Crab complex appears to include an unpulsed ultra-soft component of spectral index ∼ −4.3 which dominates the total emission below 100 MeV. This component is consistent with the expected emission from the tail end of the Crab nebula synchrotron emission. Subject headings: gamma rays: observations — pulsars: individual (Crab, Geminga, Vela)