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In the study of cosmic rays, the measurement of the energy spectrum of the primaries is one of the main issues and provides fundamental information on the most energetic phenomena in the Universe. At ultrahigh energies, beyond 1018 eV, the cosmic rays are studied by the two largest observatories built so far, the Pierre Auger Observatory and the Telescope Array. Both observatories are based on a hybrid design and reported a measurement of the energy spectrum using the high duty cycle of the surface detector and the calorimetric estimation of the energy scale provided by the fluorescence detector. The differences among the reported spectra are scrutinized by a working group made by members of the Pierre Auger and Telescope Array Collaborations. The two measurements have been found well in agreement below 1019 eV while, at higher energies, they show an energy-dependent difference that is beyond the systematic uncertainties associated to the energy scale. In this contribution we review the status and perspectives of the working group activities including new studies aiming at addressing the impact on the flux measurement at the highest energies of potential biases in the estimation of the shower size. [ABSTRACT FROM AUTHOR]

Extensive air showers initiate the fluorescence emissions from nitrogen molecules in air. The UV-light is emitted isotropically and can be used for observing the longitudinal development of extensive air showers in the atmosphere over tenth of kilometers. This measurement technique is well-established since it is exploited for many decades by several cosmic ray experiments. However, a fundamental aspect of the air shower analyses is the description of the fluorescence emission in dependence on varying atmospheric conditions. Different fluorescence yields affect directly the energy scaling of air shower reconstruction. In order to explore the various details of the nitrogen fluorescence emission in air, a few experimental groups have been performing dedicated measurements over the last decade. Most of the measurements are now finished. These experimental groups have been discussing their techniques and results in a series of Air Fluorescence Workshops commenced in 2002. At the 8 th Air Fluorescence Workshop 2011, it was suggested to develop a common way of describing the nitrogen fluorescence for application to air shower observations. Here, first analyses for a common treatment of the major dependences of the emission procedure are presented. Aspects like the contributions at different wavelengths, the dependence on pressure as it is decreasing with increasing altitude in the atmosphere, the temperature dependence, in particular that of the collisional cross sections between molecules involved, and the collisional de-excitation by water vapor are discussed. [ABSTRACT FROM AUTHOR]

We present a summary of the measurements of mass sensitive parameters at the highest cosmic ray energies done by several experiments. The max> distribution as a function of energy has been measured with fluorescence telescopes by the HiRes, TA and Auger experiments and with Cherenkov light detectors by Yakutsk. The (Xmax) or the average mass (<1nA>) has been also inferred using ground detectors, such as muon and water Cherenkov detectors. We discuss the different data analyses elaborated by each collaboration in order to extract the relevant information. Special attention is given to the different approaches used in the analysis of the data measured by fluorescence detectors in order to take into account detector biases. We present a careful analysis of the stability and performance of each analysis. The results of the different experiments will be compared and the discrepancies or agreements will be quantified. [ABSTRACT FROM AUTHOR]

We measure arrival directions, energies and mass composition of ultra-high energy cosmic rays with air fluorescence detector telescopes. The longitudinal profile of the cosmic ray induced extensive air shower cascade is imaged on focal plane of the telescope camera. Here, we show an event reconstruction method to obtain the primary information from data collected by the Telescope Array Fluorescence Detectors. In particular, we report on an 'Inverse Monte Carlo (IMC)' method in which the reconstruction process searches for an optimum solution via repeated Monte Carlo simulations including characteristics of all detectors, atmospheric conditions, photon emission and scattering processes. [ABSTRACT FROM AUTHOR]

The Central Laser Facility (CLF) is the laser device which shoots the vertical laser. CLF is located at the Center of the Telescope Array (TA) experiment site. The TA has three fluorescence detectors. CLF is equidistant from three FD stations. We made the CLF simulation using the same program as the cosmic-ray simulation. Using the CLF simulation, we reconstruct the energy shot by the CLF. In this paper, we describe some results of CLF reconstruction comparing the difference of reconstructed energy between two fluorescence telescopes. [ABSTRACT FROM AUTHOR]

We report the development of a 'hybrid trigger' system for the fluorescence detectors (FDs) and the surface detectors (SDs) in the Telescope Array (TA) experiment. It is shown from our Monte-Carlo studies that geometry determination accuracies of monocular FD events are significantly improved using SD information (position at the ground level, timing of arrival, etc...). On the other hand, in a lower energy event, the trigger efficiency of SD for lower energy showers is smaller than that of FD. Therefore no SD information is available for events with energies below 1018.5 eV taken with the TA original data acquisition system. In order to collect the small SD signals, we developed an FD-driven SD triggering system: this introduces an external triggering scheme to the TA-SD using triggering signals from FD. We present the design of the system, installation and the operational stabilities in TA observation from October 2010. [ABSTRACT FROM AUTHOR]

The accuracies in determination of air shower parameters such as longitudinal profiles or primary energies with the fluorescence detection technique are strongly dependent on atmospheric conditions of the molecular and aerosol components. Moreover, air fluorescence photon yield depends on the atmospheric density, and the transparency of the air for fluorescence photons depends on the atmospheric conditions from EAS to FDs. In this paper, we describe the atmospheric monitoring system in the Telescope Array (TA experiment), and the impact of the atmospheric conditions in air shower reconstructions. The systematic uncertainties of the determination of the primary cosmic ray energies and of the measurement of depth of maximum development (Xmax) of EASs due to atmospheric variance are evaluated by Monte Carlo simulation. [ABSTRACT FROM AUTHOR]