EDP Sciences
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Volume 411, Number 1, November III 2003
Special letters issue on: first science with integral
Page(s) L389 - L394
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20031349

A&A 411, L389-L394 (2003)
DOI: 10.1051/0004-6361:20031349


IBIS preliminary results on Cygnus X-1 spectral and temporal characteristics

A. Bazzano1, A. J. Bird2, F. Capitanio1, M. Del Santo1, P. Ubertini1, A. A. Zdziarski3, G. Di Cocco4, M. Falanga5, P. Goldoni5, A. Goldwurm5, P. Laurent5, F. Lebrun5, G. Malaguti4 and A. Segreto6

1  Istituto di Astrofisica Spaziale e Fisica Cosmica IASF, Roma, Italy
2  Departement of Physics and Astronomy, Southampton University, Southampton, UK
3  Centrum Astronomiczne im. M. Kopernika, Bartycka 18, 00-716 Warszawa, Poland
4  Istituto di Astrofisica Spaziale e Fisica Cosmica IASF, Bologna, Italy
5  Service D'Astrophysique, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
6  Istituto di Astrofisica Spaziale e Fisica Cosmica IASF, Palermo, Italy

(Received 15 July 2003 / Accepted 2 September 2003)

We report preliminary results of a broadband spectral and temporal study of the black-hole binary Cyg X-1 performed with the IBIS telescope. Cyg X-1 was the first pointed celestial target of IBIS during the INTEGRAL Performance and Verification Phase, 2002 Nov.-Dec., for a total observing time of ~2 Ms in both staring and dithering mode. Here, we report on only the staring, on-axis, observation performed in a stable instrument configuration. During the observing period the source was in its characteristic low/hard state, in which a few flares and dips have been detected. The IBIS/ISGRI results demonstrate that the INTEGRAL observatory offers a unique capability for studying correlations between hardness and/or flux in different bands over a wide photon energy range. One of our new results is finding that the hardness-flux correlation changes the sign twice over the 20-220 keV; first from positive to negative at ~50 keV, and then back to positive at ~120 keV. The former change appears to be due to the spectral curvature introduced by variable Compton reflection. The latter may be due spectral pivoting.

Key words: black hole physics -- stars: individual: Cyg X-1 -- gamma rays: observations -- X-rays: binaries -- X-rays: stars -- X-rays: general

Offprint request: A. Bazzano, angela@rm.iasf.cnr.it

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