The AGILE satellite detected several episodes of transient \gamma -ray emission from Cygnus X-3 .
Cross-correlating the AGILE light curve with both X-ray and radio monitoring data , we found that the main events of \gamma -ray activity were detected while the system was in soft spectral X-ray states , that coincide with local and often sharp minima of the hard X-ray flux , a few days before intense radio outbursts .
This repetitive temporal coincidence between the \gamma -ray transient emission and spectral state changes of the source turns out to be the spectral signature of high-energy activity from this microquasar .
The \gamma -ray differential spectrum of Cygnus X-3 ( 100 MeV – 3 GeV ) , which was obtained by averaging the data collected by AGILE during the \gamma -ray events , is consistent with a power law of photon index \alpha = 2.0 ~ { } \pm~ { } 0.2 .
Finally , we examined leptonic and hadronic emission models for the \gamma -ray activity and found that both scenarios are valid .
In particular , in the leptonic model – based on inverse Compton scatterings of mildly relativistic electrons on soft photons from both the Wolf-Rayet companion star and the accretion disk – the emitting particles may also contribute to the overall hard X-ray spectrum , possibly explaining the hard non-thermal power-law tail seen during special soft X-ray states in Cygnus X-3 .