Sensitivity of JEM-EUSO to Ensemble Fluctuations in the Ultra-High Energy Cosmic Ray Flux

TL;DR

This paper investigates how the JEM-EUSO space mission can detect fluctuations in ultra-high energy cosmic ray flux caused by the discrete distribution of sources within a local cosmic volume, highlighting its potential sensitivity over time.

Contribution

It provides a quantitative analysis of JEM-EUSO's ability to detect ensemble fluctuations in cosmic ray flux based on source proximity and observation duration.

Findings

JEM-EUSO can probe ensemble fluctuations if sources are within 3 Mpc after 3 years.

After 10 years, it can detect fluctuations with sources up to 10 Mpc away.

Spectral fluctuation studies are complementary to cosmic ray anisotropy analyses.

Abstract

The flux of ultra-high energy (UHE) cosmic rays (CRs) depends on the cosmic distribution of their sources. Data from CR observations are yet inconclusive about their exact location or distribution, but provide a measure for the average local density of these emitters. Due to the discreteness of the emitters the flux is expected to show ensemble fluctuations on top of the statistical variations, a reflection of the cosmic variance. This effect is strongest for the most energetic cosmic rays due to the limited propagation distance in the cosmic radiation background and is hence a local phenomenon. In this work we study the sensitivity of the JEM-EUSO space mission to ensemble fluctuations on the assumption of uniform distribution of sources, with local source density \sim 10^{-5} Mpc^{-3}. We show that in 3 years of observation JEM-EUSO will be able to probe ensemble fluctuations if the nearest sources are at 3 Mpc, and that after 10 years orbiting the Earth, this pathfinder mission will become sensitive to ensemble fluctuations if the nearest sources are 10 Mpc away. The study of spectral fluctuations from the local source distributions are complementary to but independent of cosmic ray anisotropy studies.