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The nature of the hard state of Cygnus X-3

Hjalmarsdotter, L. ; Zdziarski, A. A. ; Larsson, S. ; Beckmann, V. ; McCollough, M. ; Hannikainen, D. C. ; Vilhu, O.

In: Monthly Notices of the Royal Astronomical Society, 2008, vol. 384, no. 1, p. 278-290

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    Titre
    • The nature of the hard state of Cygnus X-3
    Auteur
    • Hjalmarsdotter, L.. Observatory, PO Box 14, FIN-00014 University of Helsinki, Finland
    • Zdziarski, A. A.. Copernicus Astronomical Centre, Bartycka 18, 00-716 Warszawa, Poland
    • Larsson, S.. Stockholm Observatory, Department of Astronomy, AlbaNova University Centre, 10691 Stockholm, Sweden
    • Beckmann, V.. INTEGRAL Science Data Centre, Ch. d'Écogia 16, 1290 Versoix, Switzerland
    • McCollough, M.. Smithsonian Astrophysical Observatory, 60 Garden Street, MS 67, Cambridge, MA 02138-1516, USA
    • Hannikainen, D. C.. Observatory, PO Box 14, FIN-00014 University of Helsinki, Finland
    • Vilhu, O.. Observatory, PO Box 14, FIN-00014 University of Helsinki, Finland
    Type de document
    • Postprint
    Langue
    • Anglais
    Publié dans
    • Monthly Notices of the Royal Astronomical Society, 2008, vol. 384, no. 1, p. 278-290. Blackwell Publishing Ltd
    Autre version électronique
    Identifiant OAI-PMH
    • oai:doc.rero.ch:297851
    Summary
    The X-ray binary Cygnus X-3 (Cyg X-3) is a highly variable X-ray source that displays a wide range of observed spectral states. One of the main states is significantly harder than the others, peaking at ∼20 keV, with only a weak low-energy component. Due to the enigmatic nature of this object, hidden inside the strong stellar wind of its Wolf-Rayet companion, it has remained unclear whether this state represents an intrinsic hard state, with truncation of the inner disc, or whether it is just a result of increased local absorption. We study the X-ray light curves from RXTE/ASM and CGRO/BATSE in terms of distributions and correlations of flux and hardness and find several signs of a bimodal behaviour of the accretion flow that are not likely to be the result of increased absorption in a surrounding medium. Using INTEGRAL observations, we model the broad-band spectrum of Cyg X-3 in its apparent hard state. We find that it can be well described by a model of a hard state with a truncated disc, despite the low cut-off energy, provided the accreted power is supplied to the electrons in the inner flow in the form of acceleration rather than thermal heating, resulting in a hybrid electron distribution and a spectrum with a significant contribution from non-thermal Comptonization, usually observed only in soft states. The high luminosity of this non-thermal hard state implies that either the transition takes place at significantly higher L/LE than in the usual advection models, or the mass of the compact object is ≳20 M⊙, possibly making it the most-massive black hole observed in an X-ray binary in our Galaxy so far. We find that an absorption model as well as a model of almost pure Compton reflection also fit the data well, but both have difficulties explaining other results, in particular the radio/X-ray correlation