Luo, B., Brandt, W.N., Xue, Y.Q., Alexander, D.M., Brusa, M., Bauer, F.E., Comastri, A., Fabian, A.C., Gilli, R., Lehmer, B.D., Rafferty, D.A., Schneider, D.P., Vignali, C., "Revealing a Population of Heavily Obscured Active Galactic Nuclei at z ≈ 0.5-1 in the Chandra Deep Field-South", Astrophysical Journal, 2011