The Expected Angular Resolution of the JEM-EUSO Mission

Abstract

The Extreme Universe Space Observatory on board the Japanese Experiment Module ( JEM-EUSO) is a next generation observatory for the observation of ultra high energy cosmic rays (UHECR). It is being designed to be mounted on the International Space Station (ISS). From here it will monitor the earth’s atmosphere in the ultraviolet range for extended air showers (EAS). These are induced by UHECR striking nitrogen molecules when entering the atmosphere. JEM-EUSO will detect particles with energies from 1E19 eV on up to the decade of 1E20 eV. These particles have an unknown origin. Due to the large target volume monitored, JEM-EUSO will be able to record hundreds of events exceeding energies of 7E19 eV. Therefore, it is the key instrument to improve the statistics in the high energy part of the cosmic ray spectrum, where the flux of events is extremely low, by one order of magnitude. Orbiting the earth at an altitude of about 400 km JEM-EUSO will provide an all sky coverage. The high exposure in combination with a sufficient spatial resolution will enable the identification of the sources of UHECR. During the development of the mission, the JEM-EUSO Collaboration is conducting a number of pathfinder missions to demonstrate the feasibility of the envisaged experiment. Among them is the EUSO-Balloon, a downscaled version of the JEM-EUSO instrument using the same components like optics and electronics. From summer 2014 on, it will conduct a number of stratospheric flights to measure the UV background and laser generated, artificial EAS.

ESAF, the EUSO Simulation and Analysis Framework is a software package to simulate space borne UHECR missions. Within a full end-to-end approach, it is capable to take into account all physical processes occurring, when an UHECR hits the atmosphere. From the generated data, we can reconstruct the properties of the UHECR primary, its energy, incoming direction and the type of particle it was. We use ESAF to simulate the JEM-EUSO instrument and its pathfinders.

In the scope of this dissertation, we evaluate the expected angular reconstruction performance of the planned JEM-EUSO mission. We can confirm that the instrument meets the scientific requirements. Apart from the baseline instrument, a possible advanced configuration is subject to study. From our findings we can conclude an even improved performance for this instrument. A special emphasis is given to the question how atmospheric scattering affects the fluorescence light signal. We analyze the relevance of this issue and quantify to which extent it affects the angular resolution. Further investigations deal with a possible tilting of the telescope from its nadir position for an increased exposure. We analyse how the reconstruction performance changes in dependence of the tilting angle and find that for high tilting angles the instrument looses its angular resolution capabilities.