Extreme high energy proton-gamma discrimination from space observations

TL;DR

This paper explores methods for distinguishing high-energy photon and hadron cosmic ray showers from space, focusing on the $X_{max}$ parameter, to improve understanding of cosmic ray origins at energies above 10^19.5 eV.

Contribution

It introduces a statistical technique for separating photon and hadron showers using $X_{max}$ in orbital detectors, with practical estimates for photon flux limits.

Findings

Estimated upper limit for photon fraction in cosmic rays.

Identified asymmetry in discrimination efficiency based on galactic coordinates.

Applicable to future orbital detectors like JEM-EUSO.

Abstract

The origin of the highest energy cosmic rays is still unknown. At present, the major uncertainties are located at energies above $\sim 10^{19.5}$ eV, the expected beginning of the GZK suppression. This is mainly due to the low statistics available, a problem that will be addressed in unprecedented way by the upcoming orbital detectors like JEM-EUSO. The detection of very high energy photons is of great relevance for the understanding of the origin of this extreme energy cosmic rays (EECR), due to the astrophysical information content. However, their discrimination is an experimental challenge for current and future cosmic ray detectors. In this work we study the statistical separation between hadron and photon showers from space observations at energies where both, the Landau-Pomeranchuk-Migdal (LPM) effect and magnetospheric interactions are important for the development of the cascades. We base our analysis on the $X_{max}$ parameter, which is already a well known composition discrimination parameter for ground based fluorescence observatories. Our analysis applies to orbiting detectors in general. Nevertheless, we exemplify the practical utilization of our technique by estimating a general upper limit to the photon fraction in the integral flux, attainable by an ideal orbital detector with characteristics similar to JEM-EUSO. In the process we describe the resultant asymmetry in the photon-hadron discrimination efficiency in galactic coordinates.