The AGILE monitoring of Cygnus X-3: transient gamma-ray emission and spectral constraints

Abstract

We present the AGILE-GRID 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 to 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 which then cools as it propagates outwards. Cross-correlating the AGILE-GRID light curve with X-ray and radio monitoring data, we find that the main events of gamma-ray activity have been detected while the system was in soft spectral X-ray states (RXTE/ASM count rate > 3 counts/s), that coincide with local and often sharp minima of the hard X-ray flux (Swift/BAT count rate < 0.02 counts/cm^2/s), a few days before intense radio outbursts. [...] 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 radio activity. [...] 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 the Wolf-Rayet companion star and from 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 sometimes seen during special soft X-ray states in Cygnus X-3.

arXiv commentAccepted for publication in A&A on July 26, 2012

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• Gamma-ray observations of Cygnus X-1 above 100 MeV in the hard and soft states
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• A magnetic reconnection model for explaining the multi-wavelength emission of the microquasars Cyg X-1 and Cyg X-3
• The Agile Alert System For Gamma-Ray Transients
• The Second AGILE Catalog of Gamma-Ray Sources
• Hadronic Gamma-Ray and Neutrino Emission from Cygnus X-3
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• Inverse Compton Emission from Relativistic Jets in Binary Systems
• An update on Fermi-LAT transients in the Galactic plane, including strong activity of Cygnus X-3 in mid-2020
• Estimation of the neutrino flux and resulting constraints on hadronic emission models for Cyg X-3 using AGILE data
• The gamma-ray source AGL J2241+4454 as the possible counterpart of MWC 656
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• Major and Minor Flares on Cygnus X-3 Revisited
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• Cygnus X-3 as semi-hidden PeVatron
• Orbital Phase-resolved Analysis of X-ray and Gamma-ray Observations of the High-Mass Gamma-ray Binary 4FGL J1405.1-6119
• Extreme Transients in Gamma Rays