A Search for Cosmic Ray Bursts at 0.1 PeV with a Small Air Shower Array

R W Clay ,Mikhail V Medvedev,Sławomir Stuglik,Michał Niedźwiecki,Krzysztof Rzecki,Justyna Mędrala,Alok C Gupta,Katarzyna Smelcerz,Tomasz Sośnicki,Arman Tursunov,Alan R Duffy,Oleksandr Sushchov,Michał Frontczak,Łukasz Bibrzycki,Justyna Miszczyk,Olaf Bar,Gopal Bhatta,Marcin Piekarczyk,Jassimar Singh,Bartosz Łozowski,Jilberto Zamora-Saá ,N Dhital ,K Smolek ,Apoorva D Bhatt ,D Beznosko ,D Góra ,Н Буднев ,J Stasielak ,P Homola ,David Alvarez-Castillo ,T Wibig

doi:10.3390/sym14030501

Abstract

The Cosmic Ray Extremely Distributed Observatory (CREDO) pursues a global research strategy dedicated to the search for correlated cosmic rays, so-called Cosmic Ray Ensembles (CRE). Its general approach to CRE detection does not involve any a priori considerations, and its search strategy encompasses both spatial and temporal correlations, on different scales. Here we search for time clustering of the cosmic ray events collected with a small sea-level extensive air shower array at the University of Adelaide. The array consists of seven one-square-metre scintillators enclosing an area of 10 m × 19 m. It has a threshold energy ~0.1 PeV, and records cosmic ray showers at a rate of ~6 mHz. We have examined event arrival times over a period of over 2.5 years in two equipment configurations (without and with GPS timing), recording ~300 k events and ~100 k events. We determined the event time spacing distributions between individual events and the distributions of time periods which contained specific numbers of multiple events. We find that the overall time distributions are as expected for random events. The distribution which was chosen a priori for particular study was for time periods covering five events (four spacings). Overall, these distributions fit closely with expectation, but there are two outliers of short burst periods in data for each configuration. One of these outliers contains eight events within 48 s. The physical characteristics of the array will be discussed together with the analysis procedure, including a comparison between the observed time distributions and expectation based on randomly arriving events.

Highlights

It is usual to treat the arrivals of high-energy cosmic rays as intrinsically independent processes with their modulation in space and time determined only by the directions of their sources and the rotation of the Earth
A search has been made for bursts in two time series of southern hemisphere cosmic ray events with an energy threshold of about 0.1 PeV
The criterion selected for a burst in the first time series was that there should be five events within a period of 10 s with an event rate ~6 mHz and an event deadtime of 0.5 s

Summary

Introduction

It is usual to treat the arrivals of high-energy cosmic rays as intrinsically independent processes with their modulation in space and time determined only by the directions of their sources and the rotation of the Earth. This leaves a region of observational space which may not have been well investigated for observations of charged cosmic rays, that is, the possibility of short-term time correlations, often known as bursts. The CREDO project [1] has a number of facets broadly related to searching for correlations within cosmic ray arrival time series, and one thrust of that project is to investigate possible short-term time correlations.

Methods

Results

Conclusion