The Pulsar Contribution to the Gamma-Ray Background

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We estimate the contribution of Galactic pulsars, both ordinary and millisecond pulsars (MSPs), to the high-energy (>100 MeV) gamma-ray background. We pay particular attention to the high-latitude part of the background that could be confused with an extragalactic component in existing analyses that subtract a Galactic cosmic-ray model. Our pulsar population models are calibrated to the results of large-scale radio surveys and we employ a simple empirical gamma-ray luminosity calibration to the spin-down rate that provides a good fit to existing data. We find that while ordinary pulsars are expected to contribute only a fraction 10-3 of the high-latitude gamma-ray intensity (Iₓ 1x10-5 ph s-1 cm-2 sr-1), MSPs could provide a much larger contribution and even potentially overproduce it, depending on the model parameters. We explore these dependences using a range of MSP models as a guide to how gamma-ray measurements can usefully constrain the MSP population. Existing gamma-ray background measurements and source counts already rule out several models. Finally, we show how fluctuations in the gamma-ray sky can be used to distinguish between different sources of the background.