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arXiv · 1207.6288

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

G. Piano·M. Tavani·V. Vittorini·A. Trois·A. Giuliani·A. Bulgarelli·Y. Evangelista·P. Coppi·E. Del Monte·S. Sabatini·E. Striani·I. Donnarumma·D. Hannikainen·K. I. I. Koljonen·M. McCollough·G. Pooley·S. Trushkin·R. Zanin·G. Barbiellini·M. Cardillo·P. W. Cattaneo·A. W. Chen·S. Colafrancesco·M. Feroci·F. Fuschino·M. Giusti·F. Longo·A. Morselli·A. Pellizzoni·C. Pittori·G. Pucella·M. Rapisarda·A. Rappoldi·P. Soffitta·M. Trifoglio·S. Vercellone·F. Verrecchia
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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.

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G. Piano, M. Tavani, V. Vittorini, A. Trois, A. Giuliani, A. Bulgarelli, Y. Evangelista, P. Coppi, E. Del Monte, S. Sabatini, E. Striani, I. Donnarumma, D. Hannikainen, K. I. I. Koljonen, M. McCollough, G. Pooley, S. Trushkin, R. Zanin, G. Barbiellini, M. Cardillo, P. W. Cattaneo, A. W. Chen, S. Colafrancesco, M. Feroci, F. Fuschino, M. Giusti, F. Longo, A. Morselli, A. Pellizzoni, C. Pittori, G. Pucella, M. Rapisarda, A. Rappoldi, P. Soffitta, M. Trifoglio, S. Vercellone, F. Verrecchia. 2013-03-07. The AGILE monitoring of Cygnus X-3: transient gamma-ray emission and spectral constraints. https://doi.org/10.1051/0004-6361%2F201219145

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