Probing Origin of Cosmic Rays with GRAPES-3 Experiment

Ever since the discovery in 1912, in Nobel-Prize winning observation by Victor Hess, the origin of cosmic rays has remained elusive. The GRAPES-3 experiment located in Ooty, India employs a dense array of plastic scintillator detectors and a large area muon detector, designed to probe cosmic ray physics at TeV-PeV energies. Our recent measurement of the cosmic-ray proton spectrum from 50 TeV 1.3 PeV, effectively bridges observations from both space and ground-based measurements. A kink in the spectrum has been discovered at around 166 TeV which challenges our long-held belief that the cosmic ray spectrum follows a simple power-law up to the Knee energy (at ~3 PeV). It suggests the possibility that one class of sources, commonly believed to be supernova remnants, may effectively accelerate cosmic rays up to the observed kink, while another class becomes predominant beyond the kink. In another work, we have observed two hotspot regions in the sky in the distribution of cosmic ray flux, consistent with the observations from the HAWC and ARGO-YBJ experiment [2]. In this talk, I will provide an overview of the GRAPES-3 experimental setup, discuss the above mentioned results, and tell about our ongoing upgrade activities and future research directions.

References:  

[1] Evidence of a hardening in the cosmic-ray proton spectrum at around 166 TeV observed with the GRAPES-3 experiment, F. Varsi et al., Physical Review Letters 132, 051002 (2024). 

[2] Small-scale cosmic ray anisotropy observed by the GRAPES-3 experiment at TeV energies, M. Chakraborty et al., The Astrophysical Journal, 961:87 (7pp) (2024).