A LONG-TERM STUDY OF HIGH-ENERGY GAMMA-RAY EMISSION FROM THE VELA, GEMINGA, AND CRAB PULSARS

We have analyzed similar to 3 yr of data on the Vela, Geminga, and Crab pulsars in the energy range 30 MeV to 30 GeV, gathered by the EGRET detector aboard the Compton Gamma Ray Observatory, to study long-term variability in the various characteristics of high-energy gamma-ray emission by these objects. The light curves at energies greater than or equal to 100 MeV are generally constant in time. However, it is noticed that occasionally the emission of high-energy gamma rays under one of the two peaks of the Vela and Crab pulsars is significantly less than the long-term average. The ratio of background-subtracted pulsed intensities under the two peaks, P2/P1, in the Crab pulsar is consistent with both the hypotheses of a 13.5 yr sinusoidal variation and a constant (no variation) ratio. While the integral flux at E greater than or equal to 100 MeV is nearly constant in the case of the Crab pulsar, it appears to be variable in the cases of the Vela and Geminga pulsars. The slopes of the energy spectra show evidence of variability particularly for the Vela and Geminga pulsars.