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Register a new userEvidence of Intermediate-Scale Energy Spectrum Anisotropy of Cosmic Rays E$geq$10$^{19.2}$ eV with the Telescope Array Surface Detector
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An intermediate-scale energy spectrum anisotropy has been found in the arrival directions of ultra-high energy cosmic rays of energies above $10^{19.2}$ eV in the northern hemisphere, using 7 years of data from the Telescope Array surface detector. A relative energy distribution test is done comparing events inside oversampled spherical caps of equal exposure, to those outside, using the Poisson likelihood ratio. The center of maximum significance is at $9^h$$16^m$, $45^{circ}$. and has a deficit of events with energies $10^{19.2}$$leq$$E$$<$$10^{19.75}$ eV and an excess for $E$$geq$$10^{19.75}$ eV. The post-trial probability of this energy anisotropy, appearing by chance anywhere on an isotropic sky, is found by Monte Carlo simulation to be $9$$times$$10^{-5}$ ($3.74$$sigma_{global}$).
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We have searched for intermediate-scale anisotropy in the arrival directions of ultrahigh-energy cosmic rays with energies above 57~EeV in the northern sky using data collected over a 5 year period by the surface detector of the Telescope Array experiment. We report on a cluster of events that we call the hotspot, found by oversampling using 20$^circ$-radius circles. The hotspot has a Li-Ma statistical significance of 5.1$sigma$, and is centered at R.A.=146.7$^{circ}$, Dec.=43.2$^{circ}$. The position of the hotspot is about 19$^{circ}$ off of the supergalactic plane. The probability of a cluster of events of 5.1$sigma$ significance, appearing by chance in an isotropic cosmic-ray sky, is estimated to be 3.7$times$10$^{-4}$ (3.4$sigma$).
The results on ultra-high-energy cosmic rays (UHECR) mass composition obtained with the Telescope Array surface detector are presented. The analysis employs the boosted decision tree (BDT) multivariate analysis built upon 14 observables related to both the properties of the shower front and the lateral distribution function. The multivariate classifier is trained with Monte-Carlo sets of events induced by the primary protons and iron. An average atomic mass of UHECR is presented for energies $10^{18.0}-10^{20.0} mbox{eV}$. The average atomic mass of primary particles shows no significant energy dependence and corresponds to $langle ln A rangle = 2.0 pm 0.1 (stat.) pm 0.44 (syst.)$. The result is compared to the mass composition obtained by the Telescope Array with $mbox{X}_{mbox{max}}$ technique along with the results of other experiments. Possible systematic errors of the method are discussed.
The Telescope Array (TA) collaboration has measured the energy spectrum of ultra-high energy cosmic rays with primary energies above 1.6 x 10^(18) eV. This measurement is based upon four years of observation by the surface detector component of TA. The spectrum shows a dip at an energy of 4.6 x 10^(18) eV and a steepening at 5.4 x 10^(19) eV which is consistent with the expectation from the GZK cutoff. We present the results of a technique, new to the analysis of ultra-high energy cosmic ray surface detector data, that involves generating a complete simulation of ultra-high energy cosmic rays striking the TA surface detector. The procedure starts with shower simulations using the CORSIKA Monte Carlo program where we have solved the problems caused by use of the thinning approximation. This simulation method allows us to make an accurate calculation of the acceptance of the detector for the energies concerned.
In this paper, we present results obtained from the measurements of radio emission at frequency of 32 MHz with energy more than 10$^{19}$ eV. Generalized formula that describe lateral distribution and depends on main characteristic of the air showers: energy E$_0$ and depth of maximum X$_{max}$ was derived. The formula has a good agreement with data at average and large distances from shower axis. Employing the ratio of radio emission amplitude at distances 175 m and 725 m we determined the depth of maximum X$_{max}$ for air shower with energy 3.7$cdot$10$^{19}$ eV, which in our case is equal to X$_{max}$ = 769$pm$34g$cdot$cm$^{-2}$.
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 Arrays 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 $6.2pm1.1times10^{-4}$ ($3.2sigma$) 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.
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