Observation of Anisotropy of TeV Cosmic Rays with Two Years of HAWC

TL;DR

The HAWC Observatory's two-year analysis reveals energy-dependent anisotropy in TeV cosmic rays, showing large-scale dipole, quadrupole, and small-scale features, improving resolution over previous measurements.

Contribution

This study provides the first detailed anisotropy map of TeV cosmic rays over a broad energy range with enhanced resolution and precision.

Findings

Cosmic-ray anisotropy is dominated by a dipole with phase ~40°.

Dipole amplitude increases from 8×10^{-4} at 2 TeV to 14×10^{-4} at 30.3 TeV.

Presence of large-scale quadrupole and octupole moments, plus small-scale features.

Abstract

After two years of operation, the High-Altitude Water Cherenkov (HAWC) Observatory has analyzed the TeV cosmic-ray sky over an energy range between $2.0$ and $72.8$ TeV. The HAWC detector is a ground-based air-shower array located at high altitude in the state of Puebla, Mexico. Using 300 light-tight water tanks, it collects the Cherenkov light from the particles of extensive air showers from primary gamma rays and cosmic rays. This detection method allows for uninterrupted observation of the entire overhead sky (2~sr instantaneous, 8.5~sr integrated) in the energy range from a few TeV to hundreds of TeV. Like other detectors in the northern and southern hemisphere, HAWC observes an energy-dependent anisotropy in the arrival direction distribution of cosmic rays. The observed cosmic-ray anisotropy is dominated by a dipole moment with phase $\alpha\approx40^{\circ}$ and amplitude that slowly rises in relative intensity from $8\times10^{-4}$ at 2 TeV to $14\times10^{-4}$ around 30.3 TeV, above which the dipole decreases in strength. A significant large-scale ($>60^{\circ}$ in angular extent) signal is also observed in the quadrupole and octupole moments, and significant small-scale features are also present, with locations and shapes consistent with previous observations. Compared to previous measurements in this energy range, the HAWC cosmic-ray sky maps improve on the energy resolution and fit precision of the anisotropy. These data can be used in an effort to better constrain local cosmic-ray accelerators and the intervening magnetic fields.