The Pulsar Contribution to the Diffuse Galactic Gamma‐Ray Emission

Abstract

There is active interest in the extent to which unresolved γ-ray pulsars contribute to the Galactic diffuse emission, and in whether unresolved γ-ray pulsars could be responsible for the excess of diffuse Galactic emission above 1 GeV that has been observed by EGRET. The diffuse γ-ray intensity due to unresolved pulsars is directly linked to the number of objects that should be observed in the EGRET data. We can therefore use our knowledge of the unidentified EGRET sources to constrain model parameters like the pulsar birthrate and their beaming angle. This analysis is based only on the properties of the six pulsars that have been identified in the EGRET data and is independent of choice of a pulsar emission model. We find that pulsars contribute very little to the diffuse emission at lower energies, whereas above 1 GeV they can account for 18% of the observed intensity in selected regions for a reasonable number of directly observable γ-ray pulsars (~14). The latitude distribution of the diffuse emission caused by unresolved pulsars is narrower than that of the observed diffuse emission. While the excess above 1 GeV γ-ray energy is observed up to at least 6°-8° off the plane, the pulsar contribution would be small there. Thus, pulsars do significantly contribute to the diffuse Galactic γ-ray emission above 1 GeV, but they cannot be made responsible for all the discrepancies between observed intensity and model predictions in this energy range.