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

¹ INTEGRAL Science Data Centre, Chemin d'Écogia 16, 1290 Versoix, Switzerland e-mail: Volker.Beckmann@obs.unige.ch
² Observatoire Astronomique de l'Université de Genève, Chemin des Maillettes 51, 1290 Sauverny, Switzerland
³ CSST, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁴ INTEGRAL Science Operations Centre, European Space Astronomy Centre (ESAC), Apartado 50727, 28080 Madrid, Spain
⁵ Danish National Space Centre, Technical University of Denmark, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁶ Laboratorio de Astrofísica Espacial y Física Fundamental (LAEFF-INTA), POB 50727, 28080 Madrid, Spain
⁷ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Roma, via Fosso del Cavaliere 100, 00133 Roma, Italy
⁸ Dipartimento di Astronomia, Universita' degli Studi di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁹ Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, BP 4346, 31028 Toulouse Cedex 4, France
¹⁰ Astrophysics Science Division, NASA Goddard Space Flight Center, Code 661, MD 20771, USA
¹¹ Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
¹² Observatory, PO Box 14, University of Helsinki, 00014 University of Helsinki, Finland
¹³ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße, 85748 Garching, Germany

Received: 26 June 2007
Accepted: 3 August 2007

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 (  ) black body model, whereas the X-ray spectrum above 20 keV appears to be extremely low and hard (). During the transition, the radio flux rises to a level of >, 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 ).

Conclusions.The ultrasoft state apparently precedes the emission of a jet, which is apparent in the radio and hard X-ray domain.