[Paper Review] Evidence for a Supergalactic Structure of Magnetic Deflection Multiplets of Ultra-High Energy Cosmic Rays

Telescope Array Collaboration, Rasha Abbasi+142 more|arXiv (Cornell University)|May 15, 2020

Astrophysics and Cosmic Phenomena4 citations

TL;DR

This paper presents evidence for a supergalactic structure in the magnetic deflection patterns of ultra-high-energy cosmic rays (UHECRs) using ten years of Telescope Array data. It identifies a 4.1σ post-trial correlation between UHECR arrival directions and the supergalactic plane, suggesting large-scale extragalactic magnetic fields deflecting protons from sources like the starburst galaxy M82, with an estimated coherent magnetic field of ~11 nG perpendicular to the line of sight.

ABSTRACT

Evidence for a large-scale supergalactic cosmic ray multiplet (arrival directions correlated with energy) structure is reported for ultra-high energy cosmic ray (UHECR) energies above 10$^{19}$ eV using seven years of data from the Telescope Array (TA) surface detector and updated to 10 years. Previous energy-position correlation studies have made assumptions regarding magnetic field shapes and strength, and UHECR composition. Here the assumption tested is that, since the supergalactic plane is a fit to the average matter density of the local Large Scale Structure (LSS), UHECR sources and intervening extragalactic magnetic fields are correlated with this plane. This supergalactic deflection hypothesis is tested by the entire field-of-view (FOV) behavior of the strength of intermediate-scale energy-angle correlations. These multiplets are measured in spherical cap section bins (wedges) of the FOV to account for coherent and random magnetic fields. The structure found is consistent with supergalactic deflection, the previously published energy spectrum anisotropy results of TA (the hotspot and coldspot), and toy-model simulations of a supergalactic magnetic sheet. The seven year data post-trial significance of this supergalactic structure of multiplets appearing by chance, on an isotropic sky, is found by Monte Carlo simulation to be 4.2$σ$. The ten years of data post-trial significance is 4.1$σ$. Furthermore, the starburst galaxy M82 is shown to be a possible source of the TA Hotspot, and an estimate of the supergalactic magnetic field using UHECR measurements is presented.

Motivation & Objective

• To investigate whether ultra-high-energy cosmic rays (UHECRs) exhibit correlated deflection patterns aligned with the supergalactic plane.
• To determine if observed multiplets in UHECR arrival directions are consistent with large-scale extragalactic magnetic fields.
• To assess whether the most significant correlation points to a specific astrophysical source, such as M82.
• To estimate the strength of the coherent magnetic field component perpendicular to the line of sight in the direction of the TA hotspot.

Proposed method

• Analyzed ten years of Telescope Array (TA) data for angular correlations in UHECR arrival directions within spherical cap sections ('wedges').
• Used a statistical method to identify energy-angle correlations and assess their significance post-trial.
• Rotated galactic coordinates by 90° to test for symmetry and validate alignment with the supergalactic plane.
• Fitted correlation curvature to compare with theoretical expectations of supergalactic structure.
• Estimated magnetic field strength using a proton composition model and a travel length of 3.7 Mpc.
• Evaluated consistency with prior studies on extragalactic magnetic fields and source anisotropy.

Experimental results

Research questions

• RQ1 Is there a statistically significant correlation between UHECR multiplets and the supergalactic plane?
• RQ2 Does the most significant energy-angle correlation align with the TA hotspot and point toward a specific source like M82?
• RQ3 What is the estimated strength of the coherent magnetic field component perpendicular to the line of sight in the direction of the correlation?
• RQ4 How does the observed correlation compare to galactic plane alignment, and does it support extragalactic magnetic diffusion?
• RQ5 Are the results consistent with theoretical and observational estimates of extragalactic magnetic fields?

Key findings

• A 4.1σ post-trial significance was found for the correlation between UHECR multiplets and the supergalactic plane using ten years of Telescope Array data.
• The most significant single energy-angle correlation increased from 4.6σ pre-trial (7 years) to 5.1σ pre-trial (10 years), with no new wedge parameter scan.
• The correlation lies directly over the Telescope Array hotspot, and its origin is consistent with the starburst galaxy M82 as a source.
• The average coherent magnetic field component perpendicular to the line of sight is estimated at 11 nG, assuming a purely proton composition.
• The average perpendicular magnetic field correlated with the supergalactic plane is estimated at ~10 nG, assuming a 3.7 Mpc travel length and proton composition.
• A mixed composition or longer travel length would yield a smaller magnetic field estimate, consistent with other theoretical and observational constraints.