Time-average based methods for multi-angular scale analysis of cosmic-ray data

TL;DR

This paper critically reviews time-average methods used in cosmic-ray data analysis to detect medium- and small-scale anisotropies, highlighting their advantages and systematic errors affecting the interpretation of cosmic-ray arrival directions.

Contribution

It provides a comprehensive critique of time-average based techniques for analyzing cosmic-ray anisotropies, emphasizing their limitations and potential biases.

Findings

Time-average methods are nearly insensitive to signals wider than the time-window.

Such methods can underestimate true anisotropy intensities.

Fake deficit zones can appear around true excesses due to systematic errors.

Abstract

In the last decade, a number of experiments dealt with the problem of measuring the arrival direction distribution of cosmic rays, looking for information on the propagation mechanisms and the identification of their sources. Any deviation from the isotropy may be regarded to as a signature of unforeseen or unknown phenomena, mostly if well localised in the sky and occurring at low rigidity. It induced experimenters to search for excesses down to angular scale as narrow as 10 degrees, disclosing the issue of properly filtering contributions from wider structures. A solution commonly envisaged in these years based on time-average methods to determine the reference value of cosmic ray flux. Such techniques are nearly insensitive to signals wider than the time-window in use, thus allowing to focus the analysis on medium- and small-scale signals. Nonetheless, often the signal cannot be excluded in the calculation of the reference value, what induce systematic errors. The use of time-average methods recently brought to important discoveries about the medium-scale cosmic ray anisotropy, present both in the northern and southern hemisphere. It is known that the excess (or the deficit) is observed as less intense than in reality and that fake deficit zones are rendered around true excesses, because of the absolute lack of knowledge a-priori of which signal is true and which is not. This work is an attempt to critically review the use of time average-based methods for observing extended features in the cosmic-ray arrival distribution pattern.