Implementation of the first level surface detector trigger for the Pierre Auger Observatory Engineering Array

The Pierre Auger Observatory studies ultra-high energy cosmic rays in the range of 10 18 eV up to the highest energies. The apparatus reaches full sensitivity above 10 19 ev . The southern hemisphere site is currently under construction. When completed, the surface array at this site will contain 1600 water Cherenkov detector stations distributed over 3000 km 2 . Prior to proceeding with full-scale construction, a prototype “Engineering Array”, consisting of 40 detector stations was tested. This paper describes the implementation of the first level surface detector trigger used in the engineering array.