Small-scale cosmic ray anisotropy observed by the GRAPES-3 experiment at TeV energies

Abstract

GRAPES-3 is a mid-altitude (2200 m) and near equatorial ($11.4^{\circ }$ North) air shower array, overlapping in its field of view for cosmic ray observations with experiments that are located in Northern and Southern hemispheres. We analyze a sample of $3.7\times 10^9$ cosmic ray events collected by the GRAPES-3 experiment between 1 January 2013 and 31 December 2016 with a median energy of $\sim 16$ TeV for study of small-scale ($<60^{\circ }$) angular scale anisotropies. We observed two structures labeled as A and B, deviate from the expected isotropic distribution of cosmic rays in a statistically significant manner. Structure `A' spans $50^{\circ }$ to $80^{\circ }$ in the right ascension and $-15^{\circ }$ to $30^{\circ }$ in the declination coordinate. The relative excess observed in the structure A is at the level of $(6.5\pm 1.3)\times 10^{-4}$ with a statistical significance of 6.8 standard deviations. Structure `B' is observed in the right ascension range of $110^{\circ }$ to $140^{\circ }$. The relative excess observed in this region is at the level of $(4.9\pm 1.4)\times 10^{-4}$ with a statistical significance of 4.7 standard deviations. These structures are consistent with those reported by Milagro, ARGO-YBJ, and HAWC. These observations could provide a better understanding of the cosmic ray sources, propagation and the magnetic structures in our Galaxy.