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) diffuse 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 cosmicray 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 negligibly to the γ−ray background, MSPs could provide a significant fraction of the high-latitude intensity (IX ∼ 1 × 10−5 ph s−1 cm−2 sr−1) and even potentially overproduce it, depending on the model parameters. We explore these dependences using a wide 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. We find that the maximum spin rate of MSPs is a key parameter and that for given population and γ−ray luminosity models, the γ−ray background can be used to put an upper limit on this parameter. Upcoming Fermi observations will be able to test the assumptions of our models and constrain the Galactic MSP population. We show how fluctuations in the γ−ray sky can be used to distinguish between different sources of the background. Finally, we recommend that future blind searches be extended as much as possible to cover millisecond periods and orbital acceleration in binary systems, since numerous MSPs could be important contributors to the cumulative γ−ray flux.