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

Abstract

Using the first 3.5 years of observations from the Energetic Gamma Ray Telescope (EGRET) on board the Compton Gamma Ray Observatory, phase-resolved analyses are performed on the emission from the three brightest high-energy 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 ultrasoft 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. © 1998. The American Astronomical Society. All rights reserved.