Volume 411, Number 1, November III 2003
Special letters issue on: first science with integral
|Page(s)||L389 - L394|
|Published online||17 November 2003|
Letter to the Editor
IBIS preliminary results on Cygnus X-1 spectral and temporal characteristics*
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
Corresponding author: A. Bazzano, email@example.com
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
© ESO, 2003
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