\label{fig:ttbar} The $\ttbar$ cross section at the production threshold, for a top quark mass of 174 GeV, as a function of the centre-of-mass energy, taken from Ref.~\cite{cite:1303.3758}. (Note: the measured top quark mass from Tevatron and LHC is approximately 1 GeV smaller. The 1s peak is therefore around 346 GeV instead of 348 GeV as shown here.) The black curve is the next-to-next-to-leading-order QCD-corrected cross section. The green curve shows the effect of photon emission exclusively by initial state radiation (ISR), as is expected in TLEP collisions. For illustration, the red curve includes in addition the effects of the ILC beamstrahlung at ${\sqrt{s}=350}$ GeV. The blue curve shows what would be the cross section in an hypothetical situation with beamstrahlung only, if it were possible to switch the ISR off.