Validating the improved angular resolution of the GRAPES-3 air shower array by observing the Moon shadow in cosmic rays

Academic Article


  • The Moon blocks cosmic rays, causing a deficit in their flux from its direction. Characterizing this Moon shadow is a technique used by cosmic ray air shower experiments to calibrate their angular resolution and validate the pointing accuracy. The GRAPES-3 air shower array, located in Ooty, India consists of an array of scintillator detectors and a large area muon telescope. It is sensitive to the measurement of cosmic ray and gamma ray induced showers in the TeV-PeV energy range. The timing measurements of the scintillator detectors were improved after upgrading the time-to-digital converters and coaxial cables in late 2012. The propagation delay of photomultiplier signal in coaxial cables were accurately determined on hourly basis using a random walk technique. The correction of shower front curvature for its dependence on the shower size and age together with accurate timing measurements led to a better angular resolution estimated using array division methods reported elsewhere [Jhansi et al., J. Cosmol. Astropart. Phys. 07 (2020) 024JCAPBP1475-751610.1088/1475-7516/2020/07/024]. In this paper, we discuss the validation of the angular resolution by observing the shadow of the Moon in cosmic ray flux using 3 years (2014 to 2016) of air shower data recorded during the postupgrade period. The angular resolution of the array was estimated to be 0.83°±0.09° with a statistical significance of 9.1σ and pointing accuracy along the right ascension and declination directions were obtained to be 0.032°±0.004° and 0.09°±0.003° for showers of energy >5 TeV, containing about 95% of triggered showers. The angular resolution improves to 0.38°±0.06° and 0.29°±0.06° for energy >100 TeV and >200 TeV respectively. The results are consistent with the values obtained from array division methods and are comparable to several air shower arrays that are located at almost twice the altitude of GRAPES-3. The improved angular resolution together with the accurate pointing increases the ability of GRAPES-3 to detect multi-TeV gamma ray sources.
  • Authors

    Published In

  • Physical Review D  Journal
  • Digital Object Identifier (doi)

    Pubmed Id

  • 28579305
  • Author List

  • Pattanaik D; Ahmad S; Chakraborty M; Dugad SR; Goswami UD; Gupta SK; Hariharan B; Hayashi Y; Jagadeesan P; Jain A
  • Volume

  • 106
  • Issue

  • 2