[Paper Review] Search for Anisotropy in the Ultra High Energy Cosmic Ray Spectrum using the Telescope Array Surface Detector

Rasha Abbasi, Mitsuru Abe+129 more|arXiv (Cornell University)|Jul 16, 2017

Astrophysics and Cosmic Phenomena7 citations

TL;DR

This study investigates anisotropy in the ultra high energy cosmic ray (UHECR) spectrum using the Telescope Array surface detector by comparing energy spectra in on-source (near supergalactic plane) and off-source regions. It finds a 3.2σ significant difference in spectral breaks, indicating earlier suppression in the off-source region, consistent with propagation models assuming proton primaries and negligible magnetic deflections.

ABSTRACT

The Telescope Array (TA) experiment is located in the western desert of Utah, USA, and observes ultra high energy cosmic rays (UHECRs) in the Northern hemisphere. At the highest energies, $E>10$~EeV, the shape of cosmic ray energy spectrum may carry an imprint of the source density distribution along the line of sight different in different directions of the sky. In this study, we search for such directional variations in the shape of the energy spectrum using events observed with the Telescope Array's surface detector. We divide the TA field of view into two nearly equal-exposure regions: the "on-source" region which we define as $\\pm 30^\\circ$ of the supergalactic plane containing mostly nearby structures, and the complementary "off-source" region where the sources are further away on average. We compare the UHECR spectra in these regions by fitting them to the broken power law and comparing the resulting parameters. We find that the off-source spectrum has an earlier break at highest energies. The chance probability to obtain such or larger difference in statistically equivalent distributions is estimated as $6.2\\pm1.1\ imes10^{-4}$ ($3.2\\sigma$) by a Monte-Carlo simulation. The observed difference in spectra is in a reasonable quantitative agreement with a simplified model that assumes that the UHECR sources trace the galaxy distribution from the 2MRS catalogue, primary particles are protons and the magnetic deflections can be neglected.

Motivation & Objective

• To investigate directional variations in the UHECR energy spectrum due to propagation effects along different lines of sight.
• To test whether the shape of the UHECR spectrum is modulated by source distance and large-scale structure distribution.
• To determine if observed spectral differences between sky regions can be attributed to statistical fluctuations or physical anisotropy.
• To assess the consistency of observed spectral differences with a simplified propagation model based on the 2MRS galaxy catalog.
• To explore the potential of spectral shape anisotropy as a probe for UHECR source locations and composition.

Proposed method

• Divided the Telescope Array field of view into two nearly equal-exposure regions: on-source (±30° from supergalactic plane) and off-source.
• Fitted the observed UHECR energy distributions in each region to a broken power law to extract spectral parameters.
• Used Monte-Carlo simulations to estimate the probability of observing the spectral difference under statistical equilibrium.
• Compared results with a CRPropa-based simulation assuming proton primaries, 2MRS galaxy density profile, and 0.1 nG random magnetic fields.
• Applied a simplified model of energy loss via photoproduction on cosmic microwave background radiation to predict spectral modulation.

Experimental results

Research questions

• RQ1 Is there a statistically significant difference in the energy spectrum shape between regions with nearby matter concentrations (on-source) and those without (off-source)?
• RQ2 Can the observed spectral differences be explained by statistical fluctuations or do they indicate physical anisotropy in UHECR sources?
• RQ3 To what extent do the observed spectral features match predictions from a model where UHECRs originate from galaxies traced by the 2MRS catalog?
• RQ4 How does the energy loss due to CMB interactions modulate the spectrum differently for sources at varying distances?
• RQ5 Can spectral shape anisotropy serve as a complementary method to angular clustering for identifying UHECR sources?

Key findings

• The off-source region exhibits an earlier spectral break at high energies compared to the on-source region, indicating faster flux suppression.
• The probability of observing such a spectral difference by chance in statistically equivalent distributions is estimated at 6.2 × 10⁻⁴ (3.2σ significance).
• The observed spectral difference is qualitatively consistent with a simulation assuming proton primaries and negligible magnetic deflections.
• The simulation, based on the 2MRS galaxy density profile and CRPropa 2.2.0.4, reproduces the qualitative trend of spectral modulation with distance.
• The result provides a 3.2σ indication of anisotropy in the UHECR energy spectrum, independent of angular clustering thresholds.
• The method offers a new approach to probe UHECR sources and composition by analyzing spectral shape variations across the sky.