Get the impulse response corresponding to our model.

        Returns
        -------
        psi : array_like(float)
            psi[j] is the response at lag j of the impulse response.
            We take psi[0] as unity.

def impulse_response(self, impulse_length=30):
        """
        Get the impulse response corresponding to our model.

        Returns
        -------
        psi : array_like(float)
            psi[j] is the response at lag j of the impulse response.
            We take psi[0] as unity.

        """
        from scipy.signal import dimpulse
        sys = self.ma_poly, self.ar_poly, 1
        times, psi = dimpulse(sys, n=impulse_length)
        psi = psi[0].flatten()  # Simplify return value into flat array

        return psi