A coverage independent method to analyze large scale anisotropies

TL;DR

This paper introduces a coverage-independent Fourier-based method to analyze large-scale anisotropies in cosmic ray arrival times, enabling accurate detection of sidereal modulations and correction of systematics without detailed exposure knowledge.

Contribution

The authors propose a novel Fourier expansion technique on a modified time parameter that extracts sidereal modulations independently of experimental coverage, also allowing for systematic corrections and spherical harmonic analysis.

Findings

Accurately extracts sidereal modulations without coverage knowledge

Identifies the full frequency pattern including systematics

Enables two-dimensional spherical harmonic analysis

Abstract

The arrival time distribution of cosmic ray events is well suited to extract information regarding sky anisotropies. For an experiment with nearly constant exposure, the frequency resolution one can achieve is given by the inverse of the time $T$ during which the data was recorded. For $T$ larger than one calendar year the resolution becomes sufficient to resolve the sidereal and diurnal frequencies. Using a Fourier expansion on a modified time parameter, we show in this note that one can accurately extract sidereal modulations without knowledge of the experimental coverage. This procedure also gives the full frequency pattern of the event sample under studies which contains important information about possible systematics entering in the sidereal analysis. We also show how this method allows to correct for those systematics. Finally, we show that a two dimensional analysis, in the form of the spherical harmonic ($Y_l^m$) decomposition, can be performed under the same conditions for all $m\ne 0$.