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

Abstract

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 high-energy gamma-ray pulsars, Crab, Geminga, and Vela. For each pulsar, it is found that there is detectable high-energy gamma-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 gamma-ray peaks, the hardest emission from Geminga corresponds to the second gamma-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 gamma-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.