Elsevier

Astroparticle Physics

Volume 61, February 2015, Pages 93-101
Astroparticle Physics

Energy spectrum of ultra-high energy cosmic rays observed with the Telescope Array using a hybrid technique

https://doi.org/10.1016/j.astropartphys.2014.05.002Get rights and content

Abstract

We measure the spectrum of cosmic rays with energies greater than 1018.2 eV with the fluorescence detectors (FDs) and the surface detectors (SDs) of the Telescope Array Experiment using the data taken in our first 2.3-year observation from May 27, 2008 to September 7, 2010. A hybrid air shower reconstruction technique is employed to improve accuracies in determination of arrival directions and primary energies of cosmic rays using both FD and SD data. The energy spectrum presented here is in agreement with our previously published spectra and the HiRes results.

Introduction

The Telescope Array (TA) is the largest detector of ultra-high energy cosmic rays (UHECRs) in the northern hemisphere (see Fig. 1). It is designed to explore the origin of UHECRs and the mechanisms of production, acceleration at the sources, and propagation in the inter-galactic space.

The TA [1], [2] consists of 38 fluorescence detectors (FDs) and an array of 507 surface detectors (SDs). The FDs measure longitudinal development and primary energies of air showers in the atmosphere from the amounts of light emitted by atmospheric molecules excited by charged particles in the showers [3]. The SDs measure arrival timings and local densities of the shower particles at the ground. The arrival direction and primary energy of an air shower in SD is determined from the relative timing differences of particle arrivals between SDs, and from the lateral distribution of local particle densities around the shower core, respectively [4]. The advantage of FD is that air shower energies can be determined calorimetrically knowing the fluorescence yield, which is the amount of light emitted by air molecules per total energy losses of charged particles in the showers. However there is a rather large uncertainty in arrival directions of cosmic rays determined with FD in monocular mode, in which time differences between signals of the photo-tube pixels with small angular separations are used.

A hybrid reconstruction technique, using the timing information of an SD at which air shower particles hit the ground, solves the problem. Our Monte-Carlo study shows that the inclusion of SD timing in FD monocular reconstruction significantly improves the accuracy in the determination of shower geometry (a similar method has been used in The HiRes-MIA [5] and the Pierre Auger Observatory [6]). The aim of this paper is to describe in full detail of our hybrid reconstruction method and discuss the energy spectrum of ultra-high energy cosmic rays derived from this with improved accuracies in arrival directions and primary energies. Another advantage of our strategy is that the aperture of the detector can be simply calculated from that of the SD, which is almost determined geometrically, since the SD is almost overlooked by the FD above 1019 eV.

This technique is also important to determine the composition of primary cosmic rays. Here, the FDs measure the shower development maximum in the atmosphere, Xmax, which is a parameter sensitive to the primary mass. Since this measurement is very sensitive to the shower geometry reconstruction, the hybrid technique’s improved geometrical accuracy is important. The present work on the spectrum sets the stage for subsequent publications on primary composition using the same technique.

This paper is organized as follows. We describe the TA detector in Section 2. The hybrid reconstruction method is given in Section 3. Section 4 explains air shower MC simulation and detector MC simulation. We compare the distributions of data and MC in Section 4.5, and present the energy spectrum in Section 5. The conclusion is described in Section 6.

Section snippets

The TA detectors

The TA site is located in Millard County, Utah, USA. The SD array covers an area of about 700km2. Each of the 3m2 SDs includes two layers of plastic scintillators wrapped with Tyvek reflective sheets in a stainless steel box. Scintillation photons produced by the passage of charged particles in air showers through scintillators are collected by a one-inch-diameter PhotoMultiplier Tube (PMT) for each layer. The duty cycle of the SD is nearly 100%. Full details on the SDs can be found in [7].

The

Hybrid reconstruction and event selection

The process of analysis consists of four steps: PMT selection, shower geometry reconstruction, reconstruction of longitudinal shower profile and quality cuts.

The key idea of the hybrid reconstruction is the use of timing information from one or more SDs in addition to the FD tube timings. The SD timing at which the shower plane crosses the ground gives an “anchor” in the conventional FD timing fit and significantly improves the accuracy in shower geometry determination compared to that of the

Monte-Carlo simulation of air showers and detectors

The performance of our detectors, the reconstruction programs, and the aperture are evaluated using our Monte-Carlo (MC) program. The TA MC package consists of two parts: the air shower generation part and the detector simulation part. In order to reproduce the real observation conditions in the MC, we use environmental data and calibration data that we actually measured at the site assigning a date and time for each MC event. The output of the MC simulation is written out in the same format of

Result and discussion

The energy spectrum of cosmic rays, dI/dE(E), is calculated from the number of events in an energy bin and the exposure,dI/dE(E)=n(E)E(E),where n(E) is the number of events in a given energy bin, E(E) is the energy-dependent exposure obtained from MC. Fig. 14 shows the energy spectrum measured by using 1122 events above 1018.2 eV. For comparison, the spectra of AGASA [30], HiRes [20], Auger [31], TA MD [8] and TA SD [4] are also plotted in the same figure. The TA hybrid spectrum and our

Summary

The Telescope Array including the fluorescence telescopes and the surface detector array has been fully operational since May 2008. We have developed a hybrid reconstruction technique for air showers using the longitudinal shower profile from FD and the particle arrival timing at the SD. The arrival direction and energy of an air shower can be determined with accuracies of 0.9° and 7%. These are significantly improved compared to FD monocular mode (∼5° and ∼7%). The systematic uncertainty in

Acknowledgments

The Telescope Array Experiment is supported by the Japan Society for the Promotion of Science through Grants-in-Aid for Scientific Research on Specially Promoted Research (21000002) “Extreme Phenomena in the Universe Explored by Highest Energy Cosmic Rays”, and the Inter-University Research Program of the Institute for Cosmic Ray Research; by the U.S. National Science Foundation awards PHY-0307098, PHY-0601915, PHY-0703893, PHY-0758342, PHY-0848320, PHY-1069280, and PHY-1069286 (Utah) and

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