Fine properties of symbiotic branching processes

In this work we introduce a critical curve separating the asymptotic behaviour of the moments of the symbiotic branching model, introduced by Etheridge and Fleischmann \cite{EF04}, into two regimes. Using arguments based on two different dualities and a classical result of Spitzer \cite{S58} on the exit-time of a planar Brownian motion from a wedge, we prove that the parameter governing the model provides regimes of bounded and exponentially fast growing higher moments separated by subexponential growth. The moments turn out to be closely linked to the limiting distribution as time tends to infinity. The limiting distribution can be derived by a self-duality argument extending a result of Dawson and Perkins \cite{DP98} for the mutually catalytic branching model. As an application, we show how a bound on the $18$th moment improves the result of \cite{EF04} on the speed of the interface of the symbiotic branching model