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Abstract: The Telescope Array experiment (TA) is the largest cosmic-ray detector in the northern hemi-sphere and consists of a surface detector (SD) array, plus three fluorescence detector (FD) stations overlooking the SD. The large field-of-view of an FD allows for reconstruction of the air-shower development in the atmosphere by imaging ultra-violet fluorescence light from atmospheric nitrogen excited by UHECRs. In estimation of the primary energy it is necessary to add to the calorimetric energy observed by the FD a “missing energy”, meaning the fraction of the primary energy that is not deposited by charged particles in the air. We report on the measurement of the missing energy from observed data collected by the TA FD and TA SD, independently of Monte Carlo simulations, using a technique pioneered by the Pierre Auger Observatory. We also address the effect on the energy scale attributed to fluorescence yield parameters.
Keywords: UHECR, cosmic rays, energy spectrum
Published in RUNG: 29.04.2020; Views: 3423; Downloads: 84
Full text (4,74 MB)

Abstract: Ultra high energy cosmic ray (UHECR) chemical composition is important to resolving questions about the locations of UHECR sources and propagation models. Because composition can only be deduced by a process of statistical inference via the observation of air shower maxima (Xmax), UHECR observatories with large data collection rates must be employed to reduce statistical fluctuations. Telescope Array (TA), the largest cosmic ray observatory in the Northern Hemisphere, is designed to answer the question of UHECR composition, as well as other important features of cosmic ray flux, by combining a large array of over 500 scintillation surface detectors spread over 700 km^2, and three fluorescence detector stations overlooking the array. With eight years of data recorded, results of the measurements of UHECR composition will be presented. UHECR composition is traditionally measured by comparing the first and second moments of the distributions of shower maxima, which evolves with energy, between data and simulations. Reducing statistical fluctuations in the data helps to distinguish between different primary elements in the flux. In the current generation of cosmic ray observatories, UHECR data sets are large enough, and statistical uncertainties are now small enough, that we can safely distinguish between very light primary source flux (i.e., protons) and heavy flux (i.e., iron). Reducing systematic uncertainties is also important though, since large systematic shifts in air shower maxima will influence the interpretation of the data when compared to models. TA therefore employs different methods of measuring Xmax, including stereo air fluorescence, air fluorescence-surface counter hybrid, and a new technique using only surface counters. Updated results of TA hybrid composition among the different methods are presented using up to eight years of data. Agreement among all TA hybrid composition results are shown as well as detailed systematic errors which can be further explored by comparing composition results of the different measurement methods. Comparison of TA Xmax data are compared to different composition models as well.
Keywords: UHECR, Cosmic rays, composition
Published in RUNG: 29.04.2020; Views: 3297; Downloads: 86
Full text (499,33 KB)

Abstract: Evidence of an energy dependent intermediate-scale anisotropy has been found in the arrival directions of ultra-high energy cosmic rays in the northern hemisphere, using 7 years of TA surface detector data. The previously reported ``hot spot" excess E ≥ 10^19.75 EeV is found to correspond to a deficit, or ``cold spot," of events for 10^19.2≤ E < 10^19.75 EeV. This feature suggests energy dependent magnetic deflection of cosmic-rays. The global post-trial significance of the energy spectrum deviation is found to be 3.74σ.
Keywords: UHECR, cosmic rays, energy spectrum, anisotropy, magnetic deflection
Published in RUNG: 28.04.2020; Views: 3502; Downloads: 82
Full text (4,87 MB)

Abstract: Despite deflections by Galactic and extragalactic magnetic fields, the distribution of ultra-high energy cosmic rays (UHECRs) over the celestial sphere remains a most promising observable for the identification of their sources. Thanks to a large number of detected events over the past years, a large-scale anisotropy at energies above 8 EeV has been identified, and there are also indications from the Telescope Array and Pierre Auger Collaborations of deviations from isotropy at intermediate angular scales (about 20 degrees) at the highest energies. In this contribution, we map the flux of UHECRs over the full sky at energies beyond each of two major features in the UHECR spectrum – the ankle and the flux suppression, and we derive limits for anisotropy on different angular scales in the two energy regimes. In particular, full-sky coverage enables constraints on low-order multipole moments without assumptions about the strength of higher-order multipoles. Following previous efforts from the two Collaborations, we build full-sky maps accounting for the relative exposure of the arrays and differences in the energy normalizations. The procedure relies on cross-calibrating the UHECR fluxes reconstructed in the declination band around the celestial equator covered by both observatories. We present full-sky maps at energies above ~10 EeV and ~50 EeV, using the largest datasets shared across UHECR collaborations to date. We report on anisotropy searches exploiting full-sky coverage and discuss possible constraints on the distribution of UHECR sources.
Keywords: UHECR, cosmic rays, anisotropy, Telescope Array, Pierre Auger Observatory
Published in RUNG: 28.04.2020; Views: 3420; Downloads: 86
Full text (4,92 MB)

Abstract: Evidence for the supergalactic structure of multiplets (energy-angle correlations) has previously been shown using ultra-high energy cosmic ray (UHECR) data from Telescope Array (TA) with energies above 10^19 eV. The supergalactic deflection hypothesis (that UHECR sources and intervening magnetic fields are correlated) is measured by the all-sky behavior of the strength of intermediate-scale correlations. The multiplets are measured in spherical surface wedge bins of the field-of-view to account for uniform and random magnetic fields. The structure found is consistent with the previously published energy spectrum anisotropy results of TA and toy-model simulations of a supergalactic magnetic sheet. The 7 year data post-trial significance of this feature appearing by chance, on an isotropic sky, was found by Monte Carlo simulation to be ∼4σ. The analysis has now been applied to 10 years of data.
Keywords: Cosmic rays, UHECR, energy spectrum, magnetic deflection, large-scale structure, supergalactic, multiplets
Published in RUNG: 27.04.2020; Views: 3457; Downloads: 89
Full text (1,66 MB)