A simulation study of the improvement in trigger efficiency and angularresolution of a surface array

摘要

A recently published article in Science [A. Cho, Science, 318(5852), 896 (2007)] by thePierre Auger Laboratory has helped pinpoint the source of ultrahigh energy cosmic rays, which arethe most energetic particles known in the universe. The sources found indicate support for the active galactic nuclei theory, which is that the ultrahigh energy cosmic rays come fromsupermassive black holes found at the centers of galaxies. The Pierre Auger Observatory (website:AUGER EXPERIMENT, www.auger.org) records the extensive air showers through an array of1600 particle detectors placed 1.5 km apart in a grid spread across 3000 km2. The window of TeVgamma-ray astrophysics was opened less than 2 decades ago, when the Crab Nebula was detectedfor the first time. After several years of development, the technique used by imaging atmosphericCherenkov telescopes such as HESS and MAGIC is now allowing to conduct sensitive observationsin the TeV regime. Water Cherenkov instruments such as Milagro are also providing results.Different types of extragalactic and galactic sources have been detected, showing a variety ofinteresting phenomena that are boosting theory in very high energy gamma-ray astrophysics. HereI review basic cosmic rays, the detection methods, and then present a simulation study of thegamma-ray astronomy at PeV energies—phase 3 (GRAPES3) experiment. GRAPES-3 is ascintillator detector array equipped with a muon detector tracker at Ooty, India. The capabilities ofthe array can be enhanced by changing the detector separation, etc. This can reduce the energythreshold (E_(th)), improve the angular resolution of array, and hence improve the significance thatthe array can get from a gamma-ray source. This makes the array suitable to be used for gamma-raysource detection and studies. In the present article a study of the effect of reduction in detectorseparation, on the performance of the array, using simulation techniques has been presented.Simulation shows that reducing the detector separation by a factor of 2 results in a factor of 2reduction in Eth. It also gives direction of study for further improvement in the angular resolutionand significance, specifically from a Crab Nebula-like spectrum.