Medium-scale anisotropies measured by Telescope Array surface detectors

TL;DR

The paper addresses the origin of ultra-high-energy cosmic rays by searching for medium-scale anisotropies in the TA sky. It employs an oversampling analysis with Li-Ma significance, using isotropic Monte Carlo trials and a geometrical exposure model to map excesses over fixed angular windows at multiple energy thresholds, based on 16 years of TA SD data. The analysis confirms a TA Hotspot at $(144.0^{\circ},40.5^{\circ})$ with local significance around $4.9\sigma$ (global ~$2.9\sigma$) and identifies a Perseus-Pisces excess with $S_{\rm LM}$ up to $3.7$–$3.9\sigma$ across several centers near the PPSC, aligning with nearby large-scale structures within a $150$ Mpc horizon. Cross-comparison with Auger, via exposure weighting, shows the hotspot would appear with only ~$2\sigma$ significance to Auger, consistent with its northern-sky exposure, reinforcing the importance of sky coverage in anisotropy studies. Together, these results support a link between UHECR arrival directions and the cosmic web and motivate continued observations with TAx4 to improve statistical power for source inferences and magnetic-field effects.

Abstract

The Telescope Array (TA) experiment, the largest observatory for ultra-high energy cosmic rays in the Northern Hemisphere, has identified two medium-scale anisotropies: the TA Hotspot near the constellation Ursa Major and an excess in the direction of the Perseus-Pisces supercluster. Studying these medium-scale anisotropies may provide insights into the origins of ultra-high energy cosmic rays. This presentation will explore an oversampling analysis of TA surface detector data to evaluate these medium-scale event excesses and will present the latest findings on the TA Hotspot and the Perseus-Pisces supercluster excess.

Figures (5)

• Figure 1: The sky map of the TA Hotspot using Hammer projection. The Li-Ma significance using a $25^\circ$-circle angular window is shown in equatorial coordinates for 16 years of SD events with energies greater than $5.7\times10^{19}$ eV. The black diamond indicates the maximum Li-Ma significance position measured at ($144.0^\circ,\, 40.5^\circ$). The color code indicates an excess (red) and a deficit (blue) of events compared to isotropy.
• Figure 2: The sky maps of the Perseus-Pisces Supercluster (PPSC) excess using Hammer projection. The Li-Ma significance using a $20^{\circ}$-circle angular window is shown in equatorial coordinates for different energy thresholds: (a) $E \geq 10^{19.4}$ eV, (b) $E \geq 10^{19.5}$ eV, and (c) $E \geq 10^{19.6}$ eV. The black diamonds indicate the maximum Li-Ma significance positions for each energy threshold. Additionally, (d) shows the nearby major large-scale structures (LSS) overlaid with the Li-Ma significance map for $E \geq 10^{19.4}$ eV. The color code indicates an excess (red) and a deficit (blue) of events compared to isotropy.
• Figure 3: Evolution of medium-scale anisotropies.(Left) Cumulative number of events with $E>5.7\times10^{19}$ eV (black dots) within a $25^\circ$ radius centered at the Hotspot's maximum Li-Ma significance ($144.0^\circ,\, 40.5^\circ$), compared to the isotropic expectation (orange X's). The red dashed line represents the estimated isotropic event rate, with shaded bands indicating $\pm 1\sigma$ (pink) and $\pm 2\sigma$ (orange) statistical fluctuations. (Right) Cumulative number of events with $E>10^{19.4}$ eV (black dots) within a $20^\circ$ radius around the PPSC excess center ($17.9^\circ$, $35.2^\circ$), shown alongside the isotropic expectation (orange X's) and the same statistical bands.
• Figure 4: TA and Auger exposures and exposure ratio of TA to Auger as a function of declination(Left) Exposure distributions for the Auger vertical, Auger inclined, and TA spectra, highlighting the complementary coverage of the declination band. (Right) Exposure ratio of TA to Auger as a function of declination, illustrating the mismatch in coverage.
• Figure 5: Comparison of the TA Hotspot using declination weights in equatorial coordinates.(Left) The TA Hotspot as observed by TA. (Right) The TA Hotspot as it would appear in the Auger detector, accounting for the declination weights of TA Hotspot events. The blue solid lines represent a $25^\circ$ angular distance window from the Hotspot center.