The Large Scale Cosmic-Ray Anisotropy as Observed with Milagro

TL;DR

This study analyzes seven years of Milagro observatory data to map large-scale cosmic-ray anisotropy at TeV energies, revealing a persistent deficit region and an increasing anisotropy magnitude over time.

Contribution

It provides the first detailed two-dimensional harmonic analysis of cosmic-ray anisotropy using an extensive seven-year data set from Milagro.

Findings

Detected a 0.1% anisotropy magnitude at 6 TeV.

Identified a deficit region centered at 189° right ascension.

Observed a steady increase in anisotropy over seven years.

Abstract

Results are presented of a harmonic analysis of the large scale cosmic-ray anisotropy as observed by the Milagro observatory. We show a two-dimensional display of the sidereal anisotropy pro jections in right ascension generated by the fitting of three harmonics to 18 separate declination bands. The Milagro observatory is a water Cherenkov detector located in the Jemez mountains near Los Alamos, New Mexico. With a high duty cycle and large field-of-view, Milagro is an excellent instrument for measuring this anisotropy with high sensitivity at TeV energies. The analysis is conducted using a seven year data sample consisting of more than 95 billion events, the largest such data set in existence. We observe an anisotropy with a magnitude around 0.1% for cosmic rays with a median energy of 6 TeV. The dominant feature is a deficit region of depth (2.49 +/- 0.02 stat. +/- 0.09 sys.)x10^(-3) in the direction of the Galactic North Pole centered at 189 degrees right ascension. We observe a steady increase in the magnitude of the signal over seven years.