Search for Anisotropies in Cosmic-ray Positrons Detected By the PAMELA Experiment

The Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) detector was launched on board the Russian Resurs-DK1 satellite on 2006 June 15. The 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 of 10 GV ##IMG## [http://ej.iop.org/images/0004-637X/811/1/21/apj518722ieqn1.gif] $leqslant $ R ##IMG## [http://ej.iop.org/images/0004-637X/811/1/21/apj518722ieqn2.gif] $leqslant $ 200 GV were analyzed. This article discusses the method and the results of the search for possible local sources through the 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 of δ = 0.076 at the 95% confidence level is determined. An additional search is carried out around the Sun. No evidence of an excess correlated with that direction was found.