Starting from Wightman axioms, we can define the Schwinger functions as the Wick-rotated Wightman functions (as for instance is explained in the book by R. Haag, Local Quantum Physics).
The Schwinger functions have a set of properties, which essentially come from the axioms of the original, lorentzian theory. In particular, Schwinger functions are analytic away from coincident points, as claimed in the aforementioned book.
How can we prove this claim? The book by Haag doesn't seem to explain this.