Simulation study of the detected and expected events for the EUSO-TA fluorescence detector

EUSO-TA is a ground-based cosmic ray fluorescence detector, pathfinder of the JEM-EUSO experiment, installed in 2013 at the Telescope Array (TA) site, in front of a TA fluorescence detector station, and completed with the focal surface in 2015, when it started to be operative. The data acquisition works in coincidence with TA, and this guarantees an easy identification of the cosmic ray events detected by EUSO-TA thanksto the reliability of the TA event detection and reconstruction methods. Up to now, a few cosmic ray events have been identified. Two simulation software are developed for the JEM-EUSO detectors: ESAF and Offline. ESAF is the simulation software developed in the framework of the JEM-EUSO collaboration; Offline was originally developed and successfully tested for years by the Auger Collaboration, and it has recently been adapted for the JEM-EUSO detectors. Thanks to the first events detected by EUSO-TA, ESAF and Offline have been improved in order to have a good comparison between data and simulations. This made the evaluation of the EUSO-TA detection threshold possible, with an Offline study based on the simulation of similar cosmic ray air showers as those detected and which crossed EUSO-TA FOV according to TA reconstruction. In parallel, an independent simulation of the expected number of detectable events by EUSO-TA has been conducted using ESAF code. The simulation methods are described in this work, as well as the comparison between the number of actual detected events and the number of expected ones.