The cosmic-ray anisotropy with two years of HAWC

The cosmic-ray anisotropy is, on a basic level, a well-understood phenomenon: protons and other cosmic rays produced by cosmic engines throughout the universe scatter in their paths across interstellar and local magnetic fields, resulting in a slightly inhomogeneous distribution of cosmic rays when they reach Earth from every corner of the sky.

However, this model only partially explains the features of the cosmic-ray anisotropy. A better understanding of the characteristics of this anisotropy will provide insight into the properties of the interstellar medium and can point to locations of cosmic accelerators in our galaxy producing most of the locally observed cosmic ray flux.

In a previous study, HAWC performed its first measurement of the cosmic-ray anisotropy in the energy range of 10 to 500 TeV and found a significant fine structure at the TeV scale, with an interesting feature around 45 TeV.

In a new measurement published recently in The Astrophysical Journal, HAWC has looked deeper into the region between 2 and 73 TeV and has also studied the large-scale anisotropy and the energy dependence between both the small- and large-scale features.

Relative intensity (left) and significance (right) maps showing the locations of the most significant excesses, Regions A, B, C, and the new Region D.  Image: HAWC Collaboratoin


The small-scale features are consistent with previous observations by HAWC and are described by the dominant dipole structure. The large-scale features present quadrupole and octupole patterns, which had been previously observed by other experiments.

“The dipole structure in the anisotropy is a consequence of cosmic rays being generated by an inhomogenous distribution of particle engines and then those cosmic rays zipping about the galaxy in mostly random magnetic fields,” explains Zig Hampel-Arias, who conducted most of this research as a graduate student at WIPAC. However, the predicted strength of this large-scale structure is at least ten times greater than that observed by HAWC. “And this raises questions about the assumed source distribution and the randomness of those magnetic fields,” adds Hampel-Arias. Measurements of the dipole at different energies as well as the finer details from the small-scale structure allow the study of the mechanics of the galactic cosmic-ray system, which we haven’t yet fully understood.

“The most exciting part about this study is the demonstration that both the energy and anisotropy of cosmic rays can be measured simultaneously with high precision,” says Dan Fiorino, who worked on this study as a postdoctoral researcher at the University of Maryland and who also started working on HAWC anisotropy studies as a graduate student at WIPAC. “This allows for HAWC data to be combined with IceCube data to form a daily view of the TeV cosmic-ray sky, which will reveal whether the quadrupole and octupole features are simply artifacts or true consequences of cosmic-ray propagation.”

These new measurements improve the energy resolution by 30% and have also identified new features in the anisotropy map, created with 508 uninterrupted sidereal days between May 2015 and May 2017 and 123 billion air shower events selected. This was one of the largest TeV data sets for the analysis of the cosmic-ray anisotropy and future studies with HAWC will increase the amount of data, thus providing new insights into local cosmic-ray accelerators and interstellar properties.

+ info “Observation of Anisotropy of TeV Cosmic Rays with Two Years of HAWC,” The HAWC Collaboration: A. U. Abeysekara et al. The Astrophysical Journal 865 (2018).