1. Abu-Zayyad, T. et al. (2000), Phys. Rev. Letters
84, 4276.
2. Abu-Zayyad, T. et al. (2002), e-print astro-ph/0208243.
3. Aharonian, F., Bhattacharjee, P. and Schramm, D.N. (1992), Phys. Rev. D
46, 4188.
4. Aharonian, F.A et al. (1997), Astron. Astrophys. 327, L5.
5. Aharonian, F.A. et al. (1999), Astron. Astrophys. 349, 11.