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

Abstract

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) 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 $0.83\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}0.09\ifmmode^\circ\else\textdegree\fi{}$ with a statistical significance of $9.1\ensuremath{\sigma}$ and pointing accuracy along the right ascension and declination directions were obtained to be $0.032\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}0.004\ifmmode^\circ\else\textdegree\fi{}$ and $0.09\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}0.003\ifmmode^\circ\else\textdegree\fi{}$ for showers of energy $>5\text{ }\text{ }\mathrm{TeV}$, containing about 95% of triggered showers. The angular resolution improves to $0.38\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}0.06\ifmmode^\circ\else\textdegree\fi{}$ and $0.29\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}0.06\ifmmode^\circ\else\textdegree\fi{}$ for energy $>100\text{ }\text{ }\mathrm{TeV}$ and $>200\text{ }\text{ }\mathrm{TeV}$ 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.