I am going through the Peskin and Schroeder QFT book. While proving the Gell-Mann and Low theorem in chapter 4 of their book, the authors started with the equation \begin{equation} e^{-iHT}|0\rangle = e^{-iE_{0}T}| \Omega\rangle \langle \Omega | 0 \rangle + \sum_{n\ne0}e^{-iE_{n}T}| n\rangle \langle n | 0 \rangle.\tag{p.86} \end{equation} And argued that for all the $n\ne 0$ terms die out in the limit time $T$ send to $\infty$ in a slightly imaginary direction: $T \rightarrow \infty(1-i\epsilon)$. Which yields equation (4.27) namely

\begin{equation} | \Omega \rangle = \lim_{T \rightarrow \infty(1-i\epsilon)}\frac{e^{-iHT}|0\rangle}{e^{-iE_{0}T}\langle \Omega | 0 \rangle}\tag{4.27} \end{equation} I am confused about sending the time $T$ to $\infty$ in a slightly imaginary direction: $T \rightarrow \infty(1-i\epsilon)$. Is this something mathematically rigorous or just a purely mathematical artifact? I hope someone can enlighten me making this point clear.