The contribution to the Galactic diffuse gamma-ray spectrum from unresolved rotation-powered pulsars
We consider the contribution to the Galactic diffuse gamma-ray emission from unresolved gamma-ray pulsars. Based on the thick outer gap model, Monte Carlo methods are used to simulate the properties (period, distance, magnetic field, etc.) of the Galactic population of rotation-powered pulsars the gamma-ray flux of which is lower than the threshold sensitivity of the EGRET detector on the Compton Gamma-Ray Observatory. Furthermore, the contribution to the Galactic diffuse gamma-ray spectrum from the unresolved gamma-ray pulsars is calculated. Our results indicate that the unresolved gamma-ray pulsars contribute ~5 to ~10 per cent to the measured Galactic diffuse gamma-ray emission if the birth rate of neutron stars in the Galaxy is 1 to 2 per century, and that these pulsars contribute significantly to the observed Galactic diffuse gamma-ray emission above 1 GeV. Comparing the model spectrum with the observed spectrum, we show that the unresolved gamma-ray pulsars contribute very little to the diffuse emission at lower energies but can account for ~50 per cent of the observed spectrum above 1 GeV if the product of the birth rate of neutron stars and the gamma-ray beaming fraction is about unity. Such a large pulsar contribution can explain the difference (~60 per cent) between the intensity of the Galactic diffuse emission as measured by EGRET above 1 GeV and model predictions based on cosmic ray-matter interaction only.