Carpenter K.G., J.E. Pesce, R.E. Stencel, A. Brown, S.G. Johansson, and R.F. Wing, The Astrophysical Journal Supplement Series 68, 345 (1988).
Pian E., C.M. Urry, A. Treves, L. Maraschi, S.V. Penton, J.M. Shull, J.E. Pesce, P. Grandi, T. Kii, R.I. Kollgaard, and G.M. Madejski, The Astrophysical Journal 486, 784-798 (1997).
and 
