  In this paper we study the thermalization of a spatially homogeneous system
in a strongly coupled CFT. The non-equilibrium initial state is created by
switching on a relevant perturbation in the CFT vacuum during Delta t >= t >=
-Delta t. Via AdS/CFT, the thermalization process corresponds to the
gravitational collapse of a tachyonic scalar field (m^2 = -3) in the Poincare
patch of AdS_5. In the limit Delta t < 0.02/T, the thermalization time t_T is
found to be quantitatively the same as that of a non-equilibrium state created
by a marginal perturbation discussed in Ref. [5]. In the case Delta t >= 1/T,
we also obtain double-collapse solutions but with a non-equilibrium
intermediate state at t = 0. In all the cases our results show that the system
thermalizes in a typical time t_T ~ O(1)/T. Besides, a conserved energy-moment
current in the bulk is found, which helps understand the qualitative difference
of the collapse process in the Poincare patch from that in global AdS[9, 10].
