Letter to the Editor

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

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

Corresponding author: A. Bazzano, angela@rm.iasf.cnr.it

Received: 15 July 2003
Accepted: 2 September 2003

Abstract

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.