Cygnus X-3 transition from the ultrasoft to the hard state

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Title:		Cygnus X-3 transition from the ultrasoft to the hard state
Authors:		Beckmann, V.; Soldi, S.; Bélanger, G.; Brandt, S.; Caballero-García, M. D.; de Cesare, G.; Gehrels, N.; Grebenev, S.; Vilhu, O.; von Kienlin, A.; Courvoisier, T. J.-L.
Affiliation:		AA(INTEGRAL Science Data Centre, Chemin d'Écogia 16, 1290 Versoix, Switzerland ; Observatoire Astronomique de l'Université de Genève, Chemin des Maillettes 51, 1290 Sauverny, Switzerland ; CSST, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA), AB(INTEGRAL Science Data Centre, Chemin d'Écogia 16, 1290 Versoix, Switzerland; Observatoire Astronomique de l'Université de Genève, Chemin des Maillettes 51, 1290 Sauverny, Switzerland), AC(INTEGRAL Science Operations Centre, European Space Astronomy Centre (ESAC), Apartado 50727, 28080 Madrid, Spain), AD(Danish National Space Centre, Technical University of Denmark, Juliane Maries Vej 30, 2100 Copenhagen, Denmark), AE(Laboratorio de Astrofísica Espacial y Física Fundamental (LAEFF-INTA), POB 50727, 28080 Madrid, Spain), AF(INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica di Roma, via Fosso del Cavaliere 100, 00133 Roma, Italy ; Dipartimento di Astronomia, Universita' degli Studi di Bologna, via Ranzani 1, 40127 Bologna, Italy ; Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, BP 4346, 31028 Toulouse Cedex 4, France), AG(Astrophysics Science Division, NASA Goddard Space Flight Center, Code 661, MD 20771, USA), AH(Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia), AI(Observatory, PO Box 14, University of Helsinki, 00014 University of Helsinki, Finland), AJ(Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße, 85748 Garching, Germany), AK(INTEGRAL Science Data Centre, Chemin d'Écogia 16, 1290 Versoix, Switzerland; Observatoire Astronomique de l'Université de Genève, Chemin des Maillettes 51, 1290 Sauverny, Switzerland)
Publication:		Astronomy and Astrophysics, Volume 473, Issue 3, October III 2007, pp.903-905 (A&A Homepage)
Publication Date:		10/2007
Origin:		EDP
Keywords:		stars: individual: Cyg X-3, X-rays: binaries, X-rays: individuals: Cyg X-3, stars: Wolf-Rayet, X-rays: stars, stars: binaries: close
DOI:		10.1051/0004-6361:20078160
Bibliographic Code:		2007A&A...473..903B

Abstract

Aims.The nature of Cygnus X-3 is still not understood well. This binary system might host a black hole or a neutron star. Recent observations by INTEGRAL have shown that Cygnus X-3 was again in an extremely ultrasoft state. Here we present our analysis of the transition from the ultrasoft state, dominated by blackbody radiation at soft X-rays plus non-thermal emission in the hard X-rays, to the low hard state. Methods: INTEGRAL observed Cyg X-3 six times during three weeks in late May and early June 2007. Data from IBIS/ISGRI and JEM-X1 were analysed to show the spectral transition. Results: During the ultrasoft state, the soft X-ray spectrum is well-described by an absorbed (N_H = 1.5 × 10²²~cm^-2) black body model, whereas the X-ray spectrum above 20 keV appears to be extremely low and hard (Γ ≃ 1.7). During the transition, the radio flux rises to a level of >1~Jy, and the soft X-ray emission drops by a factor of ~3, while the hard X-ray emission rises by a factor of ~14 and becomes steeper (up to Γ = 4). Conclusions: The ultrasoft state apparently precedes the emission of a jet, which is apparent in the radio and hard X-ray domain.

Based on observations with INTEGRAL, an ESA project

with instruments and science data centre funded by ESA member

states (especially the PI countries: Denmark, France, Germany,

Italy, Switzerland, Spain), Czech Republic and Poland, and with the

participation of Russia and the USA.

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