Fabian A.C., P.W. Guilbert, C. Motch, M. Ricketts, S.A. Ilovaisky, and C. Chevalier, Astronomy And Astrophysics 111, L9, L10 (1982).
Mignani R.P., S. Bagnulo, A. De Luca, G.L. Israel, G.L. Curto, C. Motch, R. Perna, N. Rea, R. Turolla, and S. Zane, in (Springer Netherlands, Dordrecht, 2007), pp. 203-210.
and 
