The angular resolution of GRAPES-3 EAS array after improved timing and shower front curvature correction based on age and size

The angular resolution of an extensive air shower (EAS) array plays a critical role in determining its sensitivity for the detection of point gamma-ray sources in the multi-TeV energy range. The GRAPES-3, an EAS array located at Ooty in India (11.4 degrees N, 76.7 degrees E, 2200 m altitude) is designed to study gamma-rays in the TeV-PeV energy range. It comprises of a dense array of 400 plastic scintillators deployed over an area of 25000 m(2) and a large area (560 m(2)) muon telescope. The development of a new statistical method has allowed the real time determination of propagation delay of each detector in the GRAPES-3 array. The shape of the shower front is known to be curved and here the details of a new method developed for accurate measurement of the shower front curvature is presented. These two developments have led to a sizable improvement in the angular resolution of the GRAPES-3 array. It is shown that the curvature depends on the size and the age of an EAS. By employing two different techniques, namely, the odd-even and the left-right methods, independent estimates of the angular resolution are obtained. The odd-even method estimates the best achievable resolution of the array. For obtaining the angular resolution, the left-right method is used after implementing the size and age dependent curvature correction. A comparison of the angular resolution as a function of EAS energy by these two methods shows them be virtually indistinguishable. The angular resolution of the GRAPES-3 array is 47' for energies E>5 TeV and improves to 17' at E>100 TeV, eventually approaching 10' at E>500 TeV.