  In this paper we discuss the existence, uniqueness and regularity of
solutions of the following system of coupled semilinear Poisson equations on a
smooth bounded domain $\Omega$ in $\mathbb{R}^n$: \[ \left\{{llll}
\mathcal{A}^s u= v^p & {\rm in} \ \ \Omega \mathcal{A}^s v = f(u) & {\rm in} \
\ \Omega u= v=0 & {\rm on} \ \ \partial\Omega \right. \] where $s\in (0, 1)$
and $\mathcal{A}^s$ denote spectral fractional Laplace operators. We assume
that $1< p<\frac{2s}{n-2s}$, and the function $f$ is superlinear and with no
growth restriction (for example $f(r)=re^r$); thus the system has a nontrivial
solution. Another important example is given by $f(r)=r^q$. In this case, we
prove that such a system admits at least one positive solution for a certain
set of the couple $(p,q)$ below the critical hyperbola \[ \frac{1}{p + 1} +
\frac{1}{q + 1} = \frac{n - 2s}{n} \] whenever $n > 2s$. For such weak
solutions, we prove an $L^\infty$ estimate of Brezis-Kato type and derive the
regularity property of the weak solutions.
