TeV flaring activity of Cygnus X-3 during 13 years

Cygnus X-3 is a peculiar X-ray binary system discovered about 40 years ago. The system has been observed throughout a wide range of the electromagnetic spectrum. It is one of the brightest Galactic X-ray sources, displaying high and low states and a rapid variability in X-rays. It is also the strongest radio source among X-ray binaries and shows both huge radio outbursts and relativistic jets. Based on the detection of ultra high energy gamma-rays, Cygnus X-3 has been suggested as one of the most powerful sources of charged cosmic ray particles in the Galaxy. The galactic source Cygnus X-3, has been regularly observed since 1995 with an average gamma-quantum flux of F(E>0.8TeV)=(6.2±0.5)×10^-13cm^-2s^-1. The binary Cyg X-3 came to a new period of flaring activity at radio- and X-ray energies in 2006. In May and July 2006 a significant increase of Cyg X-3 flux was detected by SHALON at TeV energies. Earlier, in 1997 and 2003, a comparable increase of the flux over the average value was also observed. These results provide evidence for a variability of the flux. Confirmation of the variability of very high-energy gamma radiation from Cygnus X-3 by future observations would be important for understanding the nature of this astrophysical object.