Time interval Δt_acc of the accretion process onto the NS as a function of initial NS mass M_NS for selected values of the SN core progenitor mass M_core. The horizontal dashed line is the duration of the first episode of GRB 090618, Δt = 32.5 s, which constrains the duration of the time needed by the NS to reach the critical mass. The crossing points between the dashed horizontal line and the solid curves give the NSs with M_NS that reach the critical mass in the time Δt.