Anisotropies in the gamma-ray sky from millisecond pulsars

TL;DR

This paper explores how unresolved millisecond pulsars contribute to gamma-ray sky anisotropies, providing constraints on their population and flux, and discusses how their signals can be distinguished from dark matter in anisotropy measurements.

Contribution

It introduces observationally-motivated models to constrain MSP populations using gamma-ray anisotropy data and analyzes their potential to mimic dark matter signals.

Findings

Fermi anisotropy measurements strongly constrain MSP abundance and flux.

Unresolved MSPs could significantly contribute to the gamma-ray background.

Anisotropy energy dependence can differentiate MSPs from dark matter signals.

Abstract

Pulsars emerge in the Fermi era as a sizable population of gamma-ray sources. Millisecond pulsars (MSPs) constitute an older subpopulation whose sky distribution extends to high Galactic latitudes, and it has been suggested that unresolved members of this class may contribute a significant fraction of the measured large-scale isotropic gamma-ray background (IGRB). We investigate the possible energy-dependent contribution of unresolved MSPs to the anisotropy of the Fermi-measured IGRB. For observationally-motivated MSP population models, we show that the preliminary Fermi anisotropy measurement places an interesting constraint on the abundance of MSPs in the Galaxy and the typical MSP flux, about an order of magnitude stronger than constraints on this population derived from the intensity of the IGRB alone. We also examine the possibility of a MSP component in the IGRB mimicking a dark matter signal in anisotropy-based searches, and conclude that the energy dependence of an anisotropy signature would distinguish MSPs from all but very light dark matter candidates.