code: def function(self, x, y, theta_E, gamma, center_x=0, center_y=0):
          
          gamma = self._gamma_limit(gamma)
  
          x_ = x - center_x
          y_ = y - center_y
          E = theta_E / ((3. - gamma) / 2.) ** (1. / (1. - gamma))
          # E = phi_E_spp
          eta= -gamma + 3
  
          p2 = x_**2+y_**2
          s2 = 0. # softening
          return 2 * E**2/eta**2 * ((p2 + s2)/E**2)**(eta/2)
docs: :param x: set of x-coordinates
          :type x: array of size (n)
          :param theta_E: Einstein radius of lense
          :type theta_E: float.
          :param gamma: power law slope of mass profifle
          :type gamma: <2 float
          :param q: Axis ratio
          :type q: 0<q<1
          :param phi_G: position angel of SES
          :type q: 0<phi_G<pi/2
          :returns:  function
          :raises: AttributeError, KeyError