[Paper Review] Observation of Anisotropy in the Arrival Direction Distribution of TeV Cosmic Rays with HAWC

S. BenZvi, D. W. Fiorino+1 more|arXiv (Cornell University)|Aug 19, 2015

Astrophysics and Cosmic Phenomena13 references3 citations

TL;DR

Using 181 days of data from the HAWC Observatory (2013–2014), this study measures TeV cosmic-ray anisotropy with 1° angular resolution, identifying significant small-scale excesses at (α=60°, δ=−5°), (α=120°), and (α=205°). It reveals energy-dependent spectral hardening in Region A, with a 4.2σ significant slope in relative intensity vs. energy, suggesting a northern shift in the excess at higher energies, consistent with prior Milagro and ARGO-YBJ observations.

ABSTRACT

The High-Altitude Water Cherenkov (HAWC) Observatory, located 4100 m above sea level near Sierra Negra (19$^\circ$ N) in Mexico, is sensitive to gamma rays and cosmic rays at TeV energies. The arrival direction distribution of cosmic rays at these energies shows significant anisotropy on several angular scales, with a relative intensity ranging between 10$^{-3}$ and 10$^{-4}$. We present the results of a study of cosmic-ray anisotropy based on more than 86 billion cosmic-ray air showers recorded with HAWC since June 2013. The HAWC cosmic-ray sky map, which has a median energy of 2 TeV, exhibits several regions of significantly enhanced cosmic-ray flux. We present the energy dependence of the anisotropy and the cosmic-ray spectrum in the regions of significant excess.

Motivation & Objective

• Understand the origin of small-scale anisotropy in the arrival direction distribution of TeV cosmic rays.
• Measure energy-dependent spectral behavior of cosmic-ray excess regions using high-statistics data.
• Compare HAWC results with prior northern-hemisphere experiments (Milagro, ARGO-YBJ) and southern-hemisphere data (IceCube) to identify consistency and new features.
• Characterize the morphology and energy evolution of cosmic-ray excesses, particularly in Region A, to test models of local cosmic-ray sources.
• Assess the significance of spectral hardening in excess regions to distinguish between source-driven anisotropy and scattering effects.

Proposed method

• Anisotropy is measured using 8.6×10¹⁰ cosmic-ray air showers recorded by HAWC-111 (111 water Cherenkov detectors) between June 2013 and July 2014.
• Sky maps are constructed using the HEALPix grid with 0.2° resolution to bin arrival directions into equal-area pixels.
• A reference isotropic map ⟨N⟩(α,δ) is generated via direct integration over 24-hour sidereal periods to account for detector rate variations and exposure effects.
• Residual maps are computed as the difference between observed and expected isotropic fluxes to isolate anisotropic features.
• Energy-proxy bins are created using detector simulations to estimate median energies per bin, enabling energy-dependent spectral analysis.
• Statistical significance of spectral hardening is evaluated by comparing the slope of δI vs. log(E) in Region A to slopes from 1000 random locations, with a Gaussian reference distribution.

Experimental results

Research questions

• RQ1 What is the energy-dependent behavior of cosmic-ray anisotropy in the 2 TeV median energy range as measured by HAWC?
• RQ2 Does the excess in Region A (α=60°, δ=−5°) show spectral hardening with increasing energy, and is this hardening statistically significant?
• RQ3 How do the morphological and spectral features of HAWC’s observed excesses compare with those reported by Milagro and ARGO-YBJ in the northern sky?
• RQ4 Is there evidence for a northern shift in the cosmic-ray excess in Region A at higher energies, as suggested by previous experiments?
• RQ5 Can the observed anisotropy features be consistently linked across hemispheres when compared with IceCube data at similar median energies?

Key findings

• HAWC-111 data reveal a significant small-scale excess at (α=60°, δ=−5°), consistent with prior observations by Milagro and ARGO-YBJ.
• The relative intensity in Region A shows a hardening spectrum, with a best-fit slope of (3.8±1.1)×10⁻⁴ in δI vs. log(E), 4.2σ above the background mean.
• The spectral hardening is confirmed by a reduced χ² of 1.16 for a sloped-line fit, compared to 5.66 for a horizontal line, indicating strong statistical significance.
• Region A exhibits energy-dependent morphology: the southern component (δ≈−5°) dominates below 10 TeV, while a northern component (δ≈5°) emerges at higher energies.
• Region B at α=120° shows an extended excess toward higher declinations, not fully resolved in Milagro data despite its higher latitude, and is consistent with ARGO-YBJ findings.
• A previously unconfirmed excess at α=280° in the HAWC map is observed as a sub-threshold feature, suggesting a potential new source that may be confirmed with longer data collection.