code: def derivatives(self, x, y, Rs, theta_Rs, center_x=0, center_y=0):
          
          rho0_input = self._alpha2rho0(theta_Rs=theta_Rs, Rs=Rs)
          if Rs < 0.0000001:
              Rs = 0.0000001
          x_ = x - center_x
          y_ = y - center_y
          R = np.sqrt(x_**2 + y_**2)
          f_x, f_y = self.nfwAlpha(R, Rs, rho0_input, x_, y_)
          return f_x, f_y
docs: returns df/dx and df/dy of the function (integral of NFW)