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

Abstract

The angular resolution of an extensive air shower (EAS) array plays a critical role in determining its sensitivity for the detection of point γ-ray sources in the multi-TeV energy range. The GRAPES-3, an EAS array located at Ooty in India (11.4̂N, 76.7̂E, 2200 m altitude) is designed to study γ-rays in the TeV-PeV energy range. It comprises of a dense array of 400 plastic scintillators deployed over an area of 25000 m2 and a large area (560 m2) 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.

Highlights of the results from the GRAPES-3 experiment

“…This improves to (0.29±0.06) • at >200 TeV with a significance of 3.1σ (Figure 4d). These results are consistent with the earlier analysis [6] described in the previous subsection and comparable with other experiments that are located at almost twice the altitude of GRAPES-3 (Figure 4e). We estimated the pointing accuracy of the EAS array along right ascension (α) and declination (δ) to be (0.032±0.004) • and (0.090±0.003) • , respectively (Figure 4f).…”

“…Conventionally it has been corrected by applying a constant curvature (0.215 ns•m −1 ) to the shower front, and then a planar fit is performed to estimate the EAS direction. In a recent study, it was found that the shower front curvature has a strong dependence on shower size and shower age [6]. In this work, the curvature dependences were studied in great detail and corrected to get an improved fit.…”

“…These improvements were also achieved because of better time measurement using HPTDC and real-time estimation of time offset using a statistical method called "random walk". The GRAPES-3's angular resolution is compared with other experiments such as ARGO-YBJ, Tibet ASγ, and HAWC (Figure 15 of [6]). ARGO-YBJ reported an overall angular resolution of better than 0.5 • in the energy range of 5-30 TeV [7].…”

Small-scale Cosmic-Ray Anisotropy Observed by the GRAPES-3 Experiment at TeV Energies

“…The EAS parameters such as core location, size, and age were obtained by fitting a Nishimura-Kamata-Greisen (NKG) function to the lateral distribution of particle densities (Kamata & Nishimura 1958;Greisen 1960). The directions of individual EASs in terms of zenith and azimuth angles (θ, f) were obtained by a fitting a plane front to the relative arrival time data recorded on each scintillator detector after correcting for the curvature in the shower front, which is observed to be dependent on both size and age parameters (Jhansi et al 2020;Pattanaik et al 2022). About 20% of the EAS events, mainly of low energies, could not be reconstructed properly by the NKG function.…”

A GEANT4 based simulation framework for the large area muon telescope of the GRAPES-3 experiment

“…While an initial estimate of the direction of the EAS is obtained by fitting a plane to the observed arrival times in different scintillator detectors, a more accurate estimation of direction is obtained after correcting the shower front curvature based on 𝑁 𝑒 and 𝑠 [32].…”