# Investigating an angular correlation between nearby starburst galaxies and UHECRs with the Telescope Array experiment

## TL;DR

This study tests the correlation between nearby starburst galaxies and ultra-high-energy cosmic rays using Telescope Array data, finding no significant evidence but highlighting the need for more northern hemisphere data.

## Contribution

It provides the first independent test of the Auger-reported correlation using Telescope Array data without re-optimizing model parameters.

## Key findings

- No significant correlation found ($1.1\sigma$) with current data.
- Data insufficient to confirm or refute the Auger model.
- Future expansion of TA will improve discrimination capabilities.

## Abstract

The arrival directions of cosmic rays detected by the Pierre Auger Observatory (Auger) with energies above 39~EeV were recently reported to correlate with the positions of 23 nearby starburst galaxies (SBGs): in their best-fit model, 9.7\% of the cosmic-ray flux originates from these objects and undergoes angular diffusion on a $12.9^\circ$~scale. On the other hand, some of the SBGs on their list, including the brightest one (M82), are at northern declinations outside the Auger field of view. Data from detectors in the northern hemisphere would be needed to look for cosmic-ray excesses near these objects. In this work, we tested the Auger best-fit model against data collected by the Telescope Array (TA) in a 9-year period, without trying to re-optimize the model parameters for our dataset in order not to introduce statistical penalties. The resulting test statistic (double log-likelihood ratio) was $-1.00$, corresponding to $1.1\sigma$ significance among isotropically generated random datasets, and to $-1.4\sigma$ significance among ones generated assuming the Auger best-fit model. In other words, our data is still insufficient to conclusively rule out either hypothesis. The ongoing fourfold expansion of TA will collect northern hemisphere data with much more statistics, improving our ability to discriminate between different flux models.