def grav_pot(self, x, y, rho0, Ra, Rs, center_x=0, center_y=0):
        """
        gravitational potential (modulo 4 pi G and rho0 in appropriate units)
        :param x:
        :param y:
        :param rho0:
        :param Ra:
        :param Rs:
        :param center_x:
        :param center_y:
        :return:
        """
        Ra, Rs = self._sort_ra_rs(Ra, Rs)
        x_ = x - center_x
        y_ = y - center_y
        r = np.sqrt(x_**2 + y_**2)
        pot = 4 * np.pi * rho0 * Ra ** 2 * Rs ** 2 / (Rs ** 2 - Ra ** 2) * (Rs / r * np.arctan(r / Rs) - Ra / r * np.arctan(r / Ra)
                                                                            + 1. / 2 * np.log((Rs ** 2 + r ** 2) / (Ra ** 2 + r ** 2)))
        return pot
gravitational potential (modulo 4 pi G and rho0 in appropriate units)
        :param x:
        :param y:
        :param rho0:
        :param Ra:
        :param Rs:
        :param center_x:
        :param center_y:
        :return: