I am currently reading the paper by Coleman on Symmetry breaking in 2d, which can be found here. On page 262 (4th page in the document), he is evaluating the following distribution:
$$ F_{\mu}(k)=\int d^2x\ e^{i k x} \langle0|j_\mu(x)\phi(0)|0\rangle.\tag{11b}$$
And finds since $k^\mu F_\mu = 0$, that it must be $$\sigma k_\mu\delta(k^2) \Theta(k^0) + \epsilon_{\mu\nu}k^\nu \rho(k^2) \Theta(k^0).\tag{13}$$
For some number $\sigma$ and function $\rho$.
I cannot see where-ever the $\Theta(k^0)$ part is coming from, can somebody point me the right direction?