We present the AGILE- GRID ( Astro-rivelatore Gamma a Immagini LEggero – Gamma-Ray Imaging Detector ) monitoring of Cygnus X-3 , during the period between November 2007 and July 2009 .
We report here the whole AGILE- GRID monitoring of Cygnus X-3 in the AGILE “ pointing ” mode data-taking , to confirm that the \gamma -ray activity coincides with the same repetitive pattern of multiwavelength emission and analyze in depth the overall \gamma -ray spectrum by assuming both leptonic and hadronic scenarios .
Seven intense \gamma -ray events were detected in this period , with a typical event lasting one or two days .
These durations are longer than the likely cooling times of the \gamma -ray emitting particles , implying we see continuous acceleration rather than the result of an impulsive event such as the ejection of a single plasmoid that then cools as it propagates outwards .
Cross-correlating the AGILE- GRID light curve with both X-ray and radio monitoring data , we find that the main events of \gamma -ray activity were detected while the system was in soft spectral X-ray states ( RXTE /ASM ( Rossi X-ray Timing Explorer /All-Sky Monitor ) count rate in the 3-5 keV band \gtrsim 3 ~ { } \mathrm { counts~ { } s ^ { -1 } } ) , that coincide with local and often sharp minima of the hard X-ray flux ( Swift /BAT ( Burst Alert Telescope ) count rate \lesssim 0.02 ~ { } \mathrm { counts } \mathrm { cm ^ { -2 } ~ { } s ^ { -1 } } ) , 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 \gamma -ray activity from this microquasar .
These \gamma -ray events may thus reflect a sharp transition in the structure of the accretion disk and its corona , which leads to a rebirth of the microquasar jet and subsequent enhanced activity in the radio band .
The \gamma -ray differential spectrum of Cygnus X-3 ( 100 MeV – 3 GeV ) , which was obtained by averaging the data collected by the AGILE- GRID during the \gamma -ray events , is consistent with a power law of photon index \alpha = 2.0 ~ { } \pm~ { } 0.2 .
Finally , we examine leptonic and hadronic emission models for the \gamma -ray events and find that both scenarios are valid .
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 .