Dark matter electron anisotropy: a universal upper limit

TL;DR

This paper establishes a universal upper limit on the anisotropy of high-energy cosmic ray electrons from dark matter, providing a criterion to distinguish dark matter signals from local astrophysical sources.

Contribution

It introduces a nearly model-independent upper limit on dark matter-induced electron anisotropy and analyzes the impact of dark matter sub-structures using Monte Carlo simulations.

Findings

Maximum DM anisotropy is nearly universal.

Detection of larger anisotropy indicates local astrophysical sources.

Fermi-LAT can test these anisotropy limits in upcoming years.

Abstract

We study the dipole anisotropy in the arrival directions of high energy CR electrons and positrons (CRE) of Dark Matter (DM) origin. We show that this quantity is very weakly model dependent and offers a viable criterion to discriminate among CRE from DM or from local discrete sources, like e.g. pulsars. In particular, we find that the maximum anisotropy which DM can provide is to a very good approximation a universal quantity and, as a consequence, if a larger anisotropy is detected, this would constitute a strong evidence for the presence of astrophysical local discrete CRE sources, whose anisotropy, instead, can be naturally larger than the DM upper limit. We further find that the main source of anisotropy from DM is given by the fluctuation in the number density of DM sub-structures in the vicinity of the observer and we thus devote special attention to the study of the variance in the sub-structures realization implementing a dedicated Montecarlo simulation. Such scenarios will be probed in the next years by Fermi-LAT, providing new hints, or constraints, about the nature of DM.