Operation of the Telescope Array Observatory

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The Telescope Array (TA) Observatory is the largest detector of ultra-high energy cosmic rays (UHECRs) in the northern hemisphere. An upgrade to the observatory (TAx4) is currently underway that will double the number of deployed surface detectors (SD) over an increased total instrumented area of approximately 1800 square-km. An array of florescence detectors (FDs) overlooks this SD array, providing important additional information on the detected events. This award supports the cosmic ray physics group at the University of Utah for one year of running the TA Observatory complex. The award supports the team, including managerial, engineering and technical staff, who have sole responsibility for facility maintenance, staffing observing shifts and handling data, and maintaining compliance with Bureau of Land Management land use regulations. The observatory supports auxiliary and test experiments, including recently hosting a lightning mapping array. An extensive public education and outreach program coordinated through the ASPIRE project receives millions of online visits each year, and directed activities reach tens of thousands of teachers and students. Recent TA data analyses have identified a “hotspot” of UHECRs above 57 EeV in the northern sky at 3.5 sigma significance. The observatory has also seen indications of differences in the cosmic ray spectral shape when comparing events within the hotspot region to those outside, as well as an apparent declination dependence of the spectrum at the highest energies. The source of the hotspot, however, remains unknown. Inconsistencies between the measurements of TA and the Pierre Auger Observatory, the largest UHECR detector in the southern hemisphere, are also apparent at the highest energies, including in the studies of the width and shape of the Xmax distributions. The additional data collected with the operation of the TAx4 upgrade, combined with the energy calibrations of the recently installed FDs, will greatly improve the sensitivity of the instrument toward resolving these important open questions in our understanding of the highest energy particles in the Universe.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.