Medium-scale anisotropies measured by Telescope Array surface detectors

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.

The origin of ultra high energy cosmic rays (UHECR) is a long-standing mystery. The aim of the Telescope Array (TA) experiment is to reveal this by observing the spectrum, anisotropy, and mass composition by utilizing an array of surface detectors (SD) and fluorescence detectors. One of the uncertainties in UHECR observation derives from the hadronic interaction model used for air shower Monte Carlo (MC) simulations. The number of muons observed at the ground from UHECR induced air showers is expected to depend upon the composition of primary cosmic rays. The MC prediction depends also on hadronic interaction models. One may test the hadronic interaction models by comparing the measured number of muons with the MC prediction. We studied muon densities in the UHE extensive air showers by analyzing the signal of surface detector stations for highly inclined showers which should have high muon purity. A high muon purity condition is imposed that requires the geometry of the shower and relative position of the given station. The condition implies that the muons dominate the signal, i.e. contribute about $\sim$65% of the total signal. The number of particles from air showers observed on the condition of muon purity is typically 1.88 $\pm$ 0.08 (stat.) $\pm$ 0.43 (syst.) times larger than the MC prediction value using the QGSJET I$\hspace{-.1em}$I-03 model for protons. The lateral distribution of the muons also falls more slowly in the data than the MC, causing larger discrepancy at larger lateral distance. The same feature was also obtained for other hadronic models, such as QGSJET I$\hspace{-.1em}$I-04.

The bursts of high energy events observed by the telescope array surface detector

The Telescope Array Surface Detector (TASD) in western Utah is designed for the measurement of Ultra-High-Energy Cosmic Rays (UHECRs) with characteristically low flux. During its ten years of operation, however, TASD recorded at least 24 bursts of gamma-ray showers, each consisting of two or more TASD events within a single millisecond of lightning activity. After implementation of specialized lightning detectors, the bursts were further correlated with lightning leaders preceding each stroke. We conclude that these bursts are downward Terrestrial Gamma Flashes (TGFs), similar to those observed by satellite. These data constitute the majority of all data on such events recorded from the ground. We are recreating observations using Geant4 simulations to better understand the properties of downward TGFs and the acceptance statistics of TASD. Acceleration mechanisms, for example, are poorly understood. Spectra from models such as Relativistic Runaway Electron Avalanche (RREA) and Fast Negative Breakdown (FNB) are used to generate a detector response to replicate empirical properties, especially the unique waveform timing and footprints seen at TASD. This also helps characterize the TASD acceptance and aperture for TGFs in a manner similar to cosmic ray studies.

The Telescope Array (TA) is the largest ultra-high-energy cosmic-ray (UHECR) detector in the northern hemisphere. It consists of an array of 507 surface detectors (SD) covering a total 700 km2 and three fluorescence detector stations overlooking the SD array. In this proceedings, we summarize recent results on the search for directional anisotropy of UHECRs using the latest dataset collected by the TA SD array. We obtained hints of the anisotropy of the UHECRs in the northern sky from the various analyses.

The surface detector (SD) array of the Telescope Array (TA) experiment, covering an area of 700 km$^2$ with 507 plastic scintillators, observes ultra-high-energy cosmic rays (UHECRs). This study evaluates systematic uncertainties in the energy spectrum measured by the TA SD array, focusing on the impact of hadronic interaction models and primary mass composition. Additionally, we introduce a Bayesian forward-folding framework that incorporates various systematic effects into spectral reconstruction and illustrate the influence of spectral assumptions and composition modeling.

Ultra-high energy cosmic rays (UHECRs) are extremely rare and of unknown origin. They are studied indirectly via air showers, but the reconstructed energy scale carries intrinsic uncertainties whose causes are not fully understood. We investigate the lateral distribution of particle densities in UHECR-induced showers simulated for the Telescope Array (TA) surface detectors using CORSIKA with QGSJET-II04, EPOS-LHC, and Sibyll 2.3e. Analyzing detector-level simulation outputs without reconstruction eliminates biases from the array layout and algorithms. The results show systematically steeper lateral distributions than the empirical function used in TA analysis. While 𝑆(800) remains a robust energy estimator, differences from the currently used look-up table are attributed to idealized versus realistic array assumptions. Hybrid events help calibrate this offset. Dependence on primary particle and interaction model is marginal. These findings support future efforts to derive average lateral distributions from data to constrain related uncertainties.

The Telescope Array experiment is the largest hybrid detector to observe Ultra-High Energy Cosmic Rays in the northern hemisphere. The observation started in November 2007 for Fluorescence Detector (FD) and in March 2008 for Surface Detectors (SD). Here, we present the preliminary results of the energy spectrum and mass composition of the UHECRs measured by the FD and hybrid technique from the Telescope Array three year observations. The energy spectrum measured by the Middle Drum FD station, which is the refurbished HiRes-I detector is consistent with the results from HiRes. The energy spectrum with the two newly constructed FDs and SD is also in good agreement with the result from HiRes, especially for the energy scale. The mass composition study with the slant depth of the maximum shower development (Xmax) is obtained by using the stereo and hybrid analysis. The result of the mass composition is consistent with the proton prediction.

The Telescope Array experiment is the largest hybrid detector to observe ultra-high energy cosmic rays in the northern hemisphere. The observation started in Novem- ber 2007 for Fluorescence Detector (FD) and in March 2008 for Surface Detectors (SD). Here we present the preliminary results of energy spectra measured by three different meth- ods, mass composition, photon search and AGN correlation search from the Telescope Array. The energy spectra mea- sured by the Middle-Drum FD station, which is the refur- bished detector of the High Resolution Fly's Eye (HiRes) experiment, and hybrid technique with two new FDs and SD were consistent with the result of HiRes. The system- atic error of the energies in hybrid measurement is 19%. The SD spectrum showed the suppression above 10 19.75 eV with 3.5 away from the continuous spectrum. The mea- sured mass composition with stereo technique of new FDs was consistent with pure proton model between 10 18.6 eV to 10 19.3 eV. Photon search resulted in an improved flux limit above 10 19 eV. The result of AGN correlation from TA was consistent with background.

Telescope Array (TA) is the largest ultrahigh-energy cosmic-ray (UHECR) observatory in the northern hemisphere. It explores the origin of UHECRs using a surface detector (SD) array, covering approximately 700 km2, and a fluorescence detector (FD) stations viewing the skies above the array. TA has found evidence for a cluster of cosmic rays with energies greater than 57 EeV known as a HotSpot. Recently, TA has obtained additional evidence for an excess of cosmic rays with energies above 1019.4eV and also found implications of spectrum anisotropy. An expanded SD array and FD stations called TAx4 experiment was designed to increase the data collection rate at the highest energies to confirm the anisotropies with more data. We have constructed and deployed more than half of the SDs for the TAx4 experiment and began stable data taking operations. We present TAx4 SD’s current status.

The Pierre Auger Observatory (Auger) and the Telescope Array (TA) are the world’s two largest ultra-high-energy cosmic ray (UHECR) observatories. They operate in the Southern and Northern hemispheres, respectively, at similar latitudes but with distinct surface detector (SD) designs. A significant challenge in studying UHECR physics across the full sky is the apparent discrepancy in flux measurements between the two experiments. This discrepancy could arise from astrophysical differences and/or systematic effects related to their detector designs and sensitivities to extensive air shower components. To address this, the Auger@TA working group aims to cross-calibrate the two observatories with a self-triggering micro-Auger array within the TA array. This micro-array consists of eight Auger Surface Detector (SD) stations equipped with Water Cherenkov Detectors (WCDs) and AugerPrime Surface Scintillator Detectors. Seven SD stations, configured with a centered-1-PMT design, are arranged in a hexagonal pattern with one station in the center, with 1.5 km spacing, mirroring the Auger layout. The eighth station, which features a standard 3-PMT Auger station, is located in conjunction with a TA detector at the center of the hexagon, forming a triplet for high-statistics and low-uncertainty cross-calibration. A custom communication system that uses readily available components enables seamless communication between stations and remote access to each station through a central computer. The micro-array is now fully deployed, and initial data-taking is about to start. This presentation will detail the instrumentation, communication systems, central data acquisition system, expected performance of the micro-array, and preliminary results as appropriate.

Telescope Array (TA) has completed analysis of nearly nine years of data measuring the atmospheric depth of air shower maximum (Xmax) utilizing the TA surface detector array and the Black Rock Mesa and Long Ridge fluorescence detector stations. By using both the surface array and the fluorescence detector, the geometry and arrival time of air showers can be measured very precisely providing good resolution in determining Xmax. Xmax is directly related to the air shower primary particle mass and is therefore important for understanding the composition of ultra high energy cosmic rays (UHECRs). UHECR composition will help answer questions such as the distance and location of their sources. We discuss the experimental apparatus, analysis method, and Xmax data collected. We compare the energy dependent distributions of the observed data to detailed Monte Carlo simulations of four chemical species, then test which individual species are not compatible with the data through an analysis of the shapes of the distributions. We also discuss the present state of composition analysis and interpretation between the Auger and TA experiments. These are the two largest UHECR observatories in the world with large exposures and should shed light on UHECR composition.

\nIncreased event statistics will be required to definitively answer the question of the origin(s) of Ultra-High Energy Cosmic Rays (UHECR). Using current technologies however, achieving the necessary statistics may be financially and practically impossible. We describe the status and plans of the TARA project, an effort to detect Ultra-High-Energy Cosmic Rays by their forward scattered or “bistatic” radar signature. Bistatic radar holds promise as a new remote sensing technique for UHECR, without the duty cycle limitations of nitrogen fluorescence detectors. Such a technique could prove key in advancing the study of UHECR beyond the constraints of the current generation of cosmic ray observatories. TARA consists of a low-VHF television transmitter illuminating the air above the Telescope Array (TA), and a set of radio receivers on the far side of TA approximately 50 km distant from the transmitter. We have collected radar data since April 2011 using a 2 kW transmitter at 54.1 MHz. Recently, we received permission to increase our broadcast power to 40 kW and our effective radiated power (ERP) to 6 MW. On the receiver end, we are employing software-defined radio receivers and developing real-time trigger algorithms based on the expected air shower radar echo. In addition to presenting an overview of the project status and future plans, we will present the most recent results of searches for coincidences between radar echoes and Telescope Array air shower events.\n

Ultra high energy cosmic rays over $10^{19}eV$ are thought to come from very powerful objects such as Active Galatic Nucleus(AGN). However, the number of AGNs and ultra high energy cosmic rays is not large enough to experimentally declare the correlation between them. We use Watson and Mortlock’s bayesian statistical method to compare the arrival direction data of large ground arrays such as the Telescope Array experiment with AGNs in the Veron-Cety and Veron (VCV) catalog. We also test the linearity using toy monte carlo simulation. In this presentation, we present the probability that the TA’s cosmic rays come from AGNs

Telescope Array (TA) and Auger experiments reported anisotropies in the arrival direction of ultra-high-energy cosmic rays (UHECRs). In particular, Auger Collaboration reported a correlation between UHECR events and the flux model of assumed sources and suggested a contribution of starburst galaxies (SBGs) to the anisotropy of UHECRs. However, in their study, the effect of coherent deflections by the galactic magnetic field (GMF) is not taken into account. In this study, we investigated the effect of the GMF on the arrival directions of UHECRs using the cosmic ray propagation code CRPropa3. We used a backtracking technique which consists of propagating antiparticles to map the flux outside the galaxy to at the earth. We estimate the systematic effects caused by GMF in the reported likelihood analysis. We conduct likelihood analysis for mock UHECR datasets based on the flux pattern through the GMF model. We found systematic decrease of $(f_{\rm ani}, \:\theta)$ due to GMF. As prospects for the TAx4 experiment and joint analysis of Auger and TA collaborations, we develop the likelihood analysis method with the convolution of the rigidity spectrum.