Title: Cygnus X-1 in the hard state


Proposal ID: 0120102
Subject category: Compact Object
Principal investigator: Malzac
Institute: Osservatorio Astronomico di Brera

Abstract

We propose 4 observations (75 ks each) of Cygnus X-1 in the hard state. For each individual observation, the broadband coverage of INTEGRAL will enable us to study for the first time the spectral and timing characteristics of the emission above 200 keV together with the lower energy and down to 3 keV. particular, thanks to the timing capabilities of INTEGRAL, we will learn the characteristics the rapid aperiodic variability above 200 keV (power spectra, time-lags with othe renergy bands...).We will perform simultaneous radio, IR and optical observations and search for timing correlations between these wavelengths and the high energy emission. We will focus on the expected (but not detected yet) timing correlations on short time-scales (minutes, hours). The comparisons between the different observations will allow the first comprehensive study of the broad-band spectral evolution: in particular we will search for correlated changes in the spectral parameters (spectral slope,reflection amplitude, cut-off energy and shape) on time-scales > 2 days. The multi-wavelength coverage will reveal how the radio/IR/optical spectrum changes (or not) when the high energy spectrum change. The long term fluctuations of the timing properties will be studied in parallel to the spectral evolutions, with the aim of searching for spectro/timing correlations. These observations will address the following points: what is the nature of the emitting plasma e+-e- or e--p? What is the geometry of the inner accretion flow (central spherical hot corona surrounded by a cold disk, or a patchy corona atop the a cold disc)? How does the ejection process take place, and in particular what is the nature of the link between the Comptonising corona and the jet ?What is the nature of the excess above 200 keV (non-thermal processes, multi-temperature Comptonisation...) ? What is the mechanism of short term variability ?