System mass vs. projection angle for bound and unbound solutions (thick solid and thick dashed curves, respectively) of the two-body model applied to the high-velocity E group and the main cluster. Thin/black and thick/red lines refer to t = 0.1 Gyr and t = 0.5 Gyr, respectively. Labels BI_a and BI_b indicate the bound and incoming, i.e., collapsing solutions (solid curve). Labels BO and UO indicate the bound outgoing, i.e., expanding solutions and unbound outgoing solutions (solid curve going on in the dotted curve, respectively). The horizontal lines give the range of observational values of the mass system. The thin dashed curve separates bound and unbound regions according to the Newtonian criterion (above and below the thin dashed curve, respectively).