Evidence for an intermediate-scale energy spectrum anisotropy has been found in the arrival directions of ultra-high energy cosmic rays for energies greater than 1019.2 eV in the northern hemisphere using 7 years of Telescope Array surface detector data. A relative energy distribution test is done comparing events inside oversampled spherical caps of equal exposure, to those outside, using the Poisson likelihood ratio. The center of maximum significance is at 9h16m, 45°, and has a deficit of events with energies 1019.2 ≤ E < 1019.75 eV and an excess for E ≥ 1019.75 eV. The post-trial probability of this energy anisotropy, appearing by chance anywhere on an isotropic sky, is found by Monte Carlo simulation to be 9 10-5 (3.74σ global).
Bibliographical noteFunding Information:
The Telescope Array is supported by the Japan Society for the Promotion of Science (JSPS) through Grants-in-Aid for Priority Area 431, Specially Promoted Research JP21000002, Scientific Research (S) JP19104006, Specially Promoted Research JP15H05693, Scientific Research (S) JP15H05741 and for Young Scientists (A) JPH26707011; by the joint research program of the Institute for Cosmic Ray Research (ICRR) and the University of Tokyo; by the U.S. National Science Foundation awards PHY-0601915, PHY-1404495, PHY-1404502, and PHY-1607727; by the National Research Foundation of Korea (2016R1 A2B4014967, 2016R1A5A1013277, 2017K1A4A3015188, and 2017R1A2A1A05071429); by the Russian Academy of Sciences, RFBR grant 16-02-00962a (INR), IISN project No. 4.4502.13, and Belgian Science Policy under IUAP VII/37 (ULB). The foundations of Dr. Ezekiel R. and Edna Wattis Dumke, Willard L. Eccles, and George S. and Dolores Doré Eccles all helped with generous donations. The State of Utah provided support through its Economic Development Board, and University of Utah’s Office of the Vice President for Research. The experimental site became available through the cooperation of the Utah School and Institutional Trust Lands Administration (SITLA), U.S. Bureau of Land Management (BLM), and the U.S. Air Force. We appreciate the assistance of the State of Utah and Fillmore offices of the BLM in crafting the Plan of Development for the site. Patrick Shea assisted the collaboration with valuable advice. The people and officials of Millard County, Utah, have provided greatly appreciated steadfast support. We are indebted to the Millard County Road Department for maintenance of the sites roads. We are grateful for the contributions of our home institutions’ technical staffs and the University of Utah Center for High Performance Computing for the allocation of computer time.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science