EGRET observations of pulsars

Abstract

Ever since the detection of the Crab and Vela pulsars as high‐energy gamma‐ray sources, there has been speculation that many pulsars might be gamma‐ray emitters, some of which might be detectable with more sensitive instruments. Based on theoretical considerations, most interest has focused on young pulsars where a relatively high fraction of the spin down energy might be expected to be carried away in gamma rays. Millisecond pulsars have also been suggested as possible detectable gamma‐ray sources. Consequently, the Compton Gamma Ray Observatory was designed with the ability to time tag gamma rays to an accuracy of a tenth of a millisecond in absolute time. This is needed in view of the long integration times required because of the low expected fluxes. In addition, a cooperative effort has been organized with radio astronomers who are making contemporary measurements of the ephemerides of a relatively large fraction of known pulsars which might be expected to be candidate gamma‐ray ‘‘pulsars’’ based on theoretical considerations. In this way contemporary determinations of the period and period derivative necessary for the gamma‐ray analysis are available. In addition to confirming the gamma‐ray emission from the Crab and Vela pulsars, the Energetic Gamma Ray Experiment Telescope (EGRET) has the sensitivity to do pulse phase spectroscopy on strong sources such as these. The most significant discovery of a periodic source with EGRET has been the identification of Geminga with the recently discovered x‐ray pulsar. EGRET has also detected pulsed gamma‐ray emission from the recently discovered radio pulsar PSR 1706−44 (2CG 342–02). When the systematic search is completed, EGRET will provide important new tools with which to evaluate the processes giving rise to the subclass of pulsars which are also emitting gamma‐rays at the radio periods.