The contribution to the Galactic diffuse gamma-ray spectrum from unresolved rotation-powered pulsars |
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Authors: | L Zhang & K S Cheng |
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Institution: | Department of Physics, University of Hong Kong, Pokfulam Road, Hong Kong, China |
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Abstract: | We consider the contribution to the Galactic diffuse γ-ray emission from unresolved γ-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 γ-ray spectrum from the unresolved γ-ray pulsars is calculated. Our results indicate that the unresolved γ-ray pulsars contribute ~5 to ~10 per cent to the measured Galactic diffuse γ-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 γ-ray emission above 1 GeV. Comparing the model spectrum with the observed spectrum, we show that the unresolved γ-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 γ-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. |
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Keywords: | pulsars: general stars: statistics cosmic rays diffuse radiation gamma-rays: theory |
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