2008A&A...492..527M

We present INTEGRAL light curves and spectra of the black-hole binary Cyg X-1 during a bright event that occurred in 2006 September, and which was simultaneous with a detection at 0.15-1TeV energies by the MAGIC telescope. We analyse the hard X-ray emission from 18 to 700keV with the INTEGRAL data taken on 2006 September 24-26 by the IBIS and SPI instruments. These data are supplemented with RXTE All Sky Monitor data at lower energy. We present the light curves and fit the high energy spectrum with various spectral models. Despite variations in the flux by a factor of ∼2 in the 20-700keV energy band, the shape of the energy spectrum remained remarkably stable. It is very well represented by an e-folded power law with the photon index of Γ≃1.4 and a high energy cut-off at E_c≃130-140keV. The spectrum is also well described by thermal Comptonisation including a moderate reflection component, with a solid angle of the reflector of ∼0.4x2π. The temperature of the hot Comptonising electrons is kT_e∼70keV and their Thomson optical depth is τ∼2.5. These spectral properties are typical of those observed in the low/hard state. This shows that Cyg X-1 may stay in the low hard state at least up to the flux level of 2Crab, which corresponds to ∼2-3% of the Eddington luminosity. It is the first time a persistent high-mass black-hole binary is observed at a few percent of the Eddington luminosity with a stable low/hard state spectrum over a period of a few days. Such a bright hard state has so far been observed only during the rising phase of transient low-mass black-hole binaries. The TeV detection coincides with the peak of a small X-ray flare just after a very fast rise in hard X-ray flux. In contrast, the source remained undetected by MAGIC at the peak of a larger X-ray flare occurring one day later and corresponding to the maximum of the X-ray luminosity of the whole outburst. We do not find any obvious correlation between the X-ray and TeV emission.