1. Burbidge, G., 1968, Ap. J. letter, vol.154, L41.
2. Burbidge, G. R., O’Dell, S. L., 1972, Ap. J. vol. 178, 583.
3. Chu, Y., Zhu, X., Burbidge, G., and Hewitt, A., 1984, A. Ap. 138, 408.
4. Chu, Y., Fang, L.Z., and Liu, Y., 1984, Astrophys. letters, 24, 95.
5. Cowan, C. L., 1969, Nature, vol. 224, 655.