Ultra high energy cosmic rays in the north: Measurement of UHE cosmic rays with the high resolution fly's eye (HiRes) detector

The High Resolution Fly's Eye (HiRes) observatory has been collecting Ultra High Energy Cosmic Ray (UHECR) data since 1997. The experiment observes cosmic ray air showers via the air fluorescence technique and consists of two observatory sites separated by 12.6 km in the western Utah desert. The two stations can each measure the cosmic rays in monocular mode. In addition, the data from the two stations can also be combined to form a stereo measurement of the air showers. The experiment measures such properties as the energy spectrum, chemical composition, and p-air cross-section of these cosmic rays. It also searches for point sources and other anisotropy. The spectrum is measured above similar to 3 x 10(17) eV and shows significant structure including the "ankle" and a steep fall off which is consistent with the expectation of the GZK threshold. [1, 2] The spectrum is inconsistent with a continuing spectrum at the 5 sigma level. The composition is measured using the X-max technique. It was found to be predominantly light and unchanging over the range from 10(18) to 3 x 10(19) eV. Finally, several different styles of searches for anisotropy in the data were performed. There are some tantalizing hints including potential correlation with BL Lac objects and the 'AGASA triplet", however these will need to be confirmed with an independent data set.