Search for anisotropies in cosmic-ray positrons detected by the PAMELA experiment

TL;DR

This study analyzes four years of PAMELA satellite data to search for large-scale anisotropies in cosmic-ray positrons, finding no significant anisotropy or excess related to the Sun, and setting upper limits on dipole anisotropy.

Contribution

It provides the first comprehensive anisotropy search for cosmic-ray positrons using PAMELA data, establishing upper limits on anisotropy and searching for local sources.

Findings

Positron arrival directions are consistent with isotropy.

No excess correlated with the Sun was observed.

A dipole anisotropy upper limit of 0.166 at 95% confidence level was set.

Abstract

The PAMELA detector was launched on board of the Russian Resurs-DK1 satellite on June 15, 2006. Data collected during the first four years have been used to search for large-scale anisotropies in the arrival directions of cosmic-ray positrons. The PAMELA experiment allows for a full sky investigation, with sensitivity to global anisotropies in any angular window of the celestial sphere. Data samples of positrons in the rigidity range 10 GV $\leq$ R $\leq$ 200 GV were analyzed. This article discusses the method and the results of the search for possible local sources through analysis of anisotropy in positron data compared to the proton background. The resulting distributions of arrival directions are found to be isotropic. Starting from the angular power spectrum, a dipole anisotropy upper limit \delta = 0.166 at 95% C.L. is determined. Additional search is carried out around the Sun. No evidence of an excess correlated with that direction was found.