Geometry reconstruction of fluorescence detectors revisited

TL;DR

This paper revisits the geometry reconstruction method for fluorescence detectors in cosmic ray experiments, analyzing assumptions and proposing corrections to improve accuracy in shower parameter estimation.

Contribution

It critically examines the assumptions in light arrival time calculations and introduces realistic corrections to enhance shower geometry reconstruction accuracy.

Findings

Corrections can reach 100 ns in light arrival time

Adjustments can improve arrival direction accuracy by 0.1 degrees

Shower maximum depth estimates can be refined by 5 g/cm^2

Abstract

The experimental technique of fluorescence light observation is used in current and planned air shower experiments that aim at understanding the origin of ultra-high energy cosmic rays. In the fluorescence technique, the geometry of the shower is reconstructed from the correlation between arrival time and incident angle of the signals detected by the telescope. The calculation of the expected light arrival time used so far in shower reconstruction codes is based on several assumptions. Particularly, it is assumed that fluorescence photons are produced instantaneously during the passage of the shower front and that the fluorescence photons propagate on a straight line with vacuum speed of light towards the telescope. We investigate the validity of these assumptions, how to correct them, and the impact on reconstruction parameters when adopting realistic conditions. Depending on the relative orientation of the shower to the telescope, corrections can reach 100 ns in expected light arrival time, 0.1 deg in arrival direction and 5 g/cm^2 in depth of shower maximum. The findings are relevant also for the case of "hybrid" observations where the shower is registered simultaneously by fluorescence and surface detectors.