Bounds on the density of sources of ultra-high energy cosmic rays from the Pierre Auger Observatory - Abreu, P.; Albuquerque, Ivone Freire da Mota e; Souza, Vitor de; Gouffon, Philippe; Peixoto, C J Todero; Tridapalli, D B (Journal of Cosmology and Astroparticle Physics, 2013)