1. F. Aharonian, A. Akhperjanian, M. Beilicke, et al., Astron. Astrophys. 403, 1 (2003).
2. T. C. Weekes (private communication) (2004).
3. B. M. Vladimirsky, Yu. L. Zyskin, A. P. Kornienko, et al., Izv. Krymsk. Astrofiz. Obs. 91, 74 (1994).
4. B. M. Vladimirsky, A. A. Stepanian, and V. P. Fomin, 13th Internat. Conf. Ray (Denver, 1973), Vol. 456.
5. J. E. Grindlay, H. F. Helmken, R. Brown Hanbury, et al., Astrophys. J. 201, 81 (1975).