2010MNRAS.406..689B

The compact X-ray binary system Cyg X-3 has been recently discovered as a source of GeV γ-rays by the AGILE and the Fermi satellites. It shows emission features in the GeV γ-rays similar to other γ-ray binaries which were also observed in the TeV γ-rays (LS5039 and LSI+61303). The question appears whether Cyg X-3 can be also detected in the TeV γ-rays by the Cherenkov telescopes.

Here we discuss this problem in detail based on the anisotropic inverse Compton (IC) pair cascade model successfully applied to TeV γ-ray binaries. We calculate the γ-ray light curves and γ-ray spectra expected from the cascade process occurring inside the Cyg X-3 binary system. It is found that the γ-ray light curves at GeV energies can be consistent with the γ-ray light curve observed by the Fermi for reasonable parameters of the orbit of the injection source of relativistic electrons. Moreover, we show that in such a model the sub-TeV γ-ray emission (above 100GeV) is expected to be below sensitivities of the present Cherenkov telescopes assuming that electrons are accelerated in Cyg X-3 to TeV energies. The next stage Cherenkov telescopes (MAGIC II, HESS II) should have the energy threshold in the range 20-30GeV, in order to have a chance to detect the signal from Cyg X-3. Otherwise, the positive detection of γ-rays at energies above a few tens of GeV requires a telescope with the sensitivity of of Crab units. We conclude that detection of sub-TeV γ-rays from Cyg X-3 by on-ground telescopes has to probably wait for the construction of the Cherenkov Telescope Array.