Find the potential \(\varphi(x, y)\) of an electrostatic field \(\mathbf{E}=\) \(=a(y \mathbf{i}+x \mathbf{j}),\) where \(a\) is a constant, \(\mathbf{i}\) and \(\mathbf{j}\) are the unit vectors of the \(x\) and \(y\) axes.