On the Cross-Correlation between the Arrival Direction of Ultra-High-Energy Cosmic Rays, BL Lacertae Objects, and EGRET Detections: A New Way to Identify EGRET Sources?

With the aim of testing recent claims for a particularly strong correlation between ultra-high-energy cosmic rays (UHECRs), observed with the AGASA and Yakutsk experiments, and a sample of BL Lacertae objects, we conduct a blind statistical assessment. We search for associations between the same set of BL Lac objects and the arrival directions of 33 relevant UHECRs observed with the Haverah Park and Volcano Ranch experiments. Within the accuracy of the angle determination, there are no positional coincidences. The probability that this null result arises as a statistical fluctuation from the strongly correlated case is less than 5%. This implies that the possible correlation between the arrival directions of UHECRs and BL Lac objects is not statistically sustained. We discuss the impact of our findings on the proposed additional connection among UHECRs, BL Lac objects, and EGRET γ-ray blazars. Recently, such an association was used as a classification technique for EGRET sources. Here we show that its main underlying hypothesis (i.e., the EGRET angular uncertainty is twice that quoted in the third EGRET catalog) grossly underestimates the goodness of existing γ-ray data.