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

¹ INAF/IAPS, via del Fosso del Cavaliere 100, 00133 Roma, Italy
e-mail: giovanni.piano@iaps.inaf.it
² CIFS-Torino, viale Settimio Severo 3, 10133 Torino, Italy
³ Dipartimento di Fisica, Università di Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
⁴ INAF/IASF-Milano, via E. Bassini 15, 20133 Milano, Italy
⁵ INAF/IASF-Bologna, via Gobetti 101, 40129 Bologna, Italy
⁶ Dipartimento di Fisica and INFN Trieste, via Valerio 2, 34127 Trieste, Italy
⁷ INFN-Pavia, via Bassi 6, 27100 Pavia, Italy
⁸ INFN-Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
⁹ INAF – Osservatorio Astronomico di Cagliari, località Poggio dei Pini, strada 54, 09012 Capoterra, Italy
¹⁰ ENEA Frascati, via E. Fermi 45, 00044 Frascati, Roma, Italy
¹¹ ASI Science Data Center (ASDC), via G. Galilei, 00044 Frascati, Roma, Italy
¹² INAF-OAR, via Frascati 33, 00040 Monte Porzio Catone, Italy
¹³ University of the Witwatersrand, School of Physics, WITS, 2050 Johannesburg, South Africa
¹⁴ INAF – IASF – Palermo, via U. La Malfa 15, 90146 Palermo, Italy
¹⁵ Department of Astronomy, Yale University, PO Box 208101, New Haven, CT 06520-8101, USA
¹⁶ Aalto University Metsähovi Radio Observatory, Metsähovintie 114 02540 Kylmälä, Finland
¹⁷ Department of Physics and Space Sciences, Florida Institute of Technology, 150 W. University Blvd., Melbourne, FL 32901, USA
¹⁸ Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, Massachusetts 02138, USA
¹⁹ Astrophysics Group, Cavendish Laboratory, 19 J. J. Thomson Avenue, Cambridge CB3 0HE, UK
²⁰ Special Astrophysical Observatory RAS, Karachaevo-Cherkassian Republic, 369169 Nizhnij Arkhyz, Russia
²¹ Departament d’Astronomia i Meteorologia, Institut de Ciències del Cosmos, Facultat de Física, 7a planta, Universitat de Barcelona Martí i Franquès 1, 08028 Barcelona, Spain

Received: 1 March 2012
Accepted: 26 July 2012

Abstract

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 γ-ray activity coincides with the same repetitive pattern of multiwavelength emission and analyze in depth the overall γ-ray spectrum by assuming both leptonic and hadronic scenarios. Seven intense γ-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 γ-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 γ-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  ≳ 3 counts s^-1), that coincide with local and often sharp minima of the hard X-ray flux (Swift/BAT (Burst Alert Telescope) count rate ≲0.02 counts cm^-2 s^-1), a few days before intense radio outbursts. This repetitive temporal coincidence between the γ-ray transient emission and spectral state changes of the source turns out to be the spectral signature of γ-ray activity from this microquasar. These γ-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 γ-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 γ-ray events, is consistent with a power law of photon index α = 2.0±0.2. Finally, we examine leptonic and hadronic emission models for the γ-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.