We have noted the most relevant section above at the end of each reference. Some references are relevant to more than one section; in this case, we have noted the first relevant section. Clearly the relevant literature is extensive, and the reader is urged to consult not only the references below but also the works cited in these references.
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D.M. Alexander, H. Aussel, F.E. Bauer, et al., 2002, ApJ, 568, L85 (Section 3.1)
D.M. Alexander, F.E. Bauer, W.N. Brandt, et al., 2003, AJ, 125, 383 (Section 3.3)
D.M. Alexander, F.E. Bauer, W.N. Brandt, et al., 2003, AJ, 126, 539 (Section 2.1)
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O. Almaini, S.E. Scott, J.S. Dunlop, et al., 2003, MNRAS, 338, 303 (Section 3.3)
A.J. Barger, L.L. Cowie, A.T. Steffen, et al., 2001, ApJ, 560, L23 (Section 3.3)
A.J. Barger, L.L. Cowie, P. Capak, et al., 2003, AJ, 126, 632 (Section 3.1)
A.J. Barger, L.L. Cowie, P. Capak, et al., 2003, ApJ, 584, L61 (Section 3.4)
F.E. Bauer, D.M. Alexander, W.N. Brandt, et al., 2002, AJ, 124, 2351 (Section 3.1)
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W.N. Brandt, A.E. Hornschemeier, D.M. Alexander, et al., 2001, AJ, 122, 1 (Section 3.1)
W.N. Brandt, D.M. Alexander, A.E. Hornschemeier, et al., 2001, AJ, 122, 2810 (Section 2.1)
W.N. Brandt, C. Vignali, D.P. Schneider, et al., 2004, Adv. Space Res., in press (astro-ph/0212082; Section 3.4)
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A. Comastri, M. Mignoli, P. Ciliegi, et al., 2002, ApJ, 571, 771 (Section 3.1)
L.L. Cowie, A.J. Barger, M.W. Bautz, et al., 2003, ApJ, 584, L57 (Section 3.2)
S. Cristiani, D.M. Alexander, F.E. Bauer, et al., 2004, ApJ, 600, L119 (Section 3.4)
S.M. Croom, R.J. Smith, B.J. Boyle, et al., 2004, MNRAS, in press (astro-ph/0403040; Section 3.2)
F. Fiore, M. Brusa, F. Cocchia, et al., 2003, A&A, 409, 79 (Section 3.2)
P. Gandhi, C.S. Crawford, A.C. Fabian, et al., 2004, MNRAS, 348, 529 (Section 3.1)
R. Giacconi, A. Zirm, J.X. Wang, et al., 2002, ApJS, 139, 369 (Section 2.1)
M. Giavalisco, H.C. Ferguson, A.M. Koekemoer, et al., 2004, ApJ, 600, L93 (Section 2.1)
R. Gilli, 2004, Adv. Space Res., in press (astro-ph/0303115; Section 3.1)
P.J. Green, J.D. Silverman, R.A. Cameron, et al., 2004, ApJS, 150, 43 (Section 2.2)
G. Hasinger, B. Altieri, M. Arnaud, et al., 2001, A&A, 365, L45 (Section 2.1)
A.E. Hornschemeier, W.N. Brandt, G.P. Garmire, et al., 2000, ApJ, 541, 49 (Section 3.3)
A.E. Hornschemeier, F.E. Bauer, D.M. Alexander, et al., 2003, AJ, 126, 575 (Section 3.1)
A.E. Hornschemeier, D.M. Alexander, F.E. Bauer, et al., 2004, ApJ, 600, L147 (Section 3.1)
M.P. Hunt, C.C. Steidel, K.L. Adelberger, et al., 2004, ApJ, in press (astro-ph/0312041; Section 3.1)
F. Jansen, D. Lumb, B. Altieri, et al., 2001, A&A, 365, L1 (Section 1)
D.W. Kim, R.A. Cameron, J.J. Drake, et al., 2004, ApJS, 150, 19 (Section 2.2)
A.M. Koekemoer, D.M. Alexander, F.E. Bauer, et al., 2004, ApJ, 600, L123 (Section 3.4)
A. Marconi, G. Risaliti, R. Gilli, et al., 2004, MNRAS, in press (astro-ph/0311619; Section 3.1)
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D.P. Schneider, X. Fan, P.B. Hall, et al., 2003, AJ, 126, 2579 (Section 3.4)
P. Severgnini, A. Caccianiga, V. Braito, et al., 2003, A&A, 406, 483 (Section 3.1)
I.A.G. Snellen, P.N. Best, 2001, MNRAS, 328, 897 (Section 3.1)
A.T. Steffen, A.J. Barger, L.L. Cowie, et al., 2003, ApJ, 596, L23 (Section 3.2)
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Y. Ueda, M. Akiyama, K. Ohta, et al., 2003, ApJ, 598, 886 (Section 3.1)
C. Vignali, F.E. Bauer, D.M. Alexander, et al., 2002, ApJ, 580, L105 (Section 3.4)
C. Vignali, W.N. Brandt, D.P. Schneider, 2003, AJ, 125, 433 (Section 3.4)
C. Vignali, W.N. Brandt, D.P. Schneider, et al., 2003, AJ, 125, 2876 (Section 3.4)
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R.E. Williams, B. Blacker, M. Dickinson, et al., 1996, AJ, 112, 1335 (Section 2.1)
C. Wolf, L. Wisotzki, A. Borch, et al., 2003, A&A, 408, 499 (Section 3.2)
D.G. York, J. Adelman, J.E. Anderson, et al., 2000, AJ, 120, 1579 (Section 3.4)
F. Yuan & R. Narayan, 2004, ApJ, in press (astro-ph/0401117; Section 3.1)