[Paper Review] A search for possible anisotropies of cosmic rays with 0.1 < E < 10 EeV in the region of the Galactic Centre

Edivaldo Moura Santos|arXiv (Cornell University)|Jun 19, 2007

Astrophysics and Cosmic Phenomena5 citations

TL;DR

This study searches for anisotropies in cosmic rays near the Galactic Centre using 3.5 years of Pierre Auger Observatory data (2004–2007), analyzing energies from 0.1 to 10 EeV. It employs Li-Ma significance tests and Gaussian/top-hat windowing to assess point-like and extended sources, finding no significant excesses and setting a 95% CL upper limit on neutron flux that excludes most theoretical models of neutron production at the Galactic Centre.

ABSTRACT

We present updated results for anisotropy searches in the direction of the Galactic Center (GC) at energies in the ranges: 0.1<E<1 EeV and 1<E<10 EeV. We use data from the Pierre Auger Observatory up to March, 2007. Present analyzes are therefore based on a substantially larger data set than our previous published results. A limit on the flux coming from a hypothetical point-like neutron source at the GC for 1<E<10 EeV was imposed, and searches for extended excesses were also performed.

Motivation & Objective

• To search for anisotropies in cosmic rays near the Galactic Centre using updated data from the Pierre Auger Observatory.
• To test for possible point-like or extended excesses of cosmic rays in the energy range 0.1 < E < 10 EeV.
• To set a 95% confidence level upper limit on the flux of hypothetical neutron sources at the Galactic Centre, particularly for 1 < E < 10 EeV.
• To assess the consistency of cosmic ray flux with isotropy in the direction of Sagittarius A* using Li-Ma significance tests and angular resolution-matched filters.
• To evaluate the implications of these results for theoretical models predicting high-energy neutron emission from the Galactic Centre.

Proposed method

• Data from the Pierre Auger Observatory's surface detector (SD) were used, requiring 3 or more triggered tanks in a compact configuration and level 5 trigger acceptance.
• Events were selected with zenith angles θ < 60° and energy estimates derived from SD signal integration, calibrated using fluorescence detector measurements.
• Li-Ma overdensity significance tests were applied to assess excesses in 5°, 10°, and 20° top-hat windows and 0.8°–1.3° Gaussian beams matched to the angular resolution (1.2°–2.0° FWHM).
• Background estimation used coverage maps accounting for detector dead time, array growth, and weather effects to model acceptance variations.
• A differential cosmic ray spectrum ΦCR(E) ∝ E−3.3 was assumed, with flux limits derived using the formula: Φ95s = (n95s / nexp) × (4πσ²) × ∫ΦCR(E) dE, where n95s is the 95% CL upper bound on signal counts.
• The flux upper limit was converted into a luminosity bound: LGC < 1.25κ × 10³⁴ erg/s, with κ = 1.2 for HiRes normalization.

Experimental results

Research questions

• RQ1 Is there a significant excess of cosmic rays in the direction of the Galactic Centre at energies between 0.1 and 10 EeV?
• RQ2 Can a hypothetical point-like neutron source at the Galactic Centre be detected with current Auger data, and what flux limits can be set?
• RQ3 Are there extended cosmic ray excesses in the Galactic Centre region, and how do they compare to isotropic expectations?
• RQ4 Do the observed cosmic ray fluxes near Sagittarius A* support or rule out theoretical models predicting high-energy neutron emission?
• RQ5 How do the results compare to earlier claims from AGASA and SUGAR, and to H.E.S.S. TeV gamma-ray observations?

Key findings

• No significant cosmic ray excess was found in the Galactic Centre region for 0.1 < E < 1 EeV, with the observed-to-expected ratio of 1.00 ± 0.02 (stat) ± 0.01 (syst) in a 1.3° Gaussian window.
• For 1 < E < 10 EeV, the observed-to-expected ratio was 0.99 ± 0.17 (stat) ± 0.01 (syst) in a 0.8° Gaussian window, consistent with isotropy.
• The 95% CL upper limit on the integrated flux of a hypothetical point-like neutron source at the Galactic Centre is Φ95s = 0.018κ km⁻² yr⁻¹, with κ = 1.2 for HiRes normalization.
• The corresponding upper limit on the source luminosity is LGC < 1.25κ × 10³⁴ erg/s, which excludes most theoretical models predicting neutron production at the Galactic Centre.
• Extended source searches over 10° and 20° windows showed no significant excesses, with observed-to-expected ratios of 1.07 ± 0.04 and 1.03 ± 0.02, respectively, for 1 < E < 10 EeV.
• The distribution of Li-Ma significance values across the Galactic Centre region is consistent with isotropy, as confirmed by comparison with Monte Carlo simulations and Gaussian fluctuations.