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The nature of the hard state of Cygnus X-3
Stockholm University, Faculty of Science, Department of Astronomy.
Stockholm University, Faculty of Science, Department of Astronomy.
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2008 (English)In: Monthly Notices of the Royal Astronomical Society, Vol. 384, no 1, 278-290 p.Article in journal (Refereed) Published
Abstract [en]

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 >~20Msolar, 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.

Place, publisher, year, edition, pages
2008. Vol. 384, no 1, 278-290 p.
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Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:su:diva-18310DOI: doi:10.1111/j.1365-2966.2007.12688.xOAI: oai:DiVA.org:su-18310DiVA: diva2:184833
Available from: 2009-01-28 Created: 2009-01-28 Last updated: 2011-01-10Bibliographically approved

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