The pulsar contribution to the gamma-ray background

We estimate the contribution of Galactic pulsars, both ordinary and millisecond pulsars (MSPs), to the high-energy ( > 100 MeV) γ−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 γ−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 γ−ray intensity ( I X ∼ 1 × 10 −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 γ−ray measurements can usefully constrain the MSP population. Existing γ−ray background measurements and source counts already rule out several models. Finally, we show how fluctuations in the γ−ray sky can be used to distinguish between different sources of the background.