The Pulsar Contribution to the Gamma-Ray Background

TL;DR

This paper estimates the contribution of Galactic pulsars, especially millisecond pulsars, to the high-energy gamma-ray background, showing MSPs could significantly influence the background and that measurements can constrain pulsar models.

Contribution

It provides a calibrated model of pulsar gamma-ray emission and explores how gamma-ray data can constrain pulsar populations and distinguish their contributions.

Findings

Ordinary pulsars contribute minimally (~10^-3) to the high-latitude gamma-ray background.

Millisecond pulsars could account for a significant or dominant portion of the background.

Gamma-ray measurements and source counts can rule out certain pulsar population models.

Abstract

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 (I_X~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.