EDP Sciences
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Volume 492, Number 2, December III 2008
Page(s) 527 - 534
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:200810227
Published online 15 October 2008

A&A 492, 527-534 (2008)
DOI: 10.1051/0004-6361:200810227

An intense state of hard X-ray emission of Cyg X-1 observed by INTEGRAL coincident with TeV measurements

J. Malzac1, P. Lubiński2, 3, A. A. Zdziarski2, M. Cadolle Bel4, M. Türler3, 5, and P. Laurent6

1  Centre d'Étude Spatiale des Rayonnements (CESR), OMP, UPS, CNRS, 9 Avenue du Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
    e-mail: malzac@cesr.fr
2  Centrum Astronomiczne im. M. Kopernika, Bartycka 18, 00-716 Warszawa, Poland
3  ISDC Data Centre for Astrophysics, Chemin d'Ecogia 16, 1290 Versoix, Switzerland
4  European Space Astronomy Centre (ESAC), Apartado/PO Box 78, Villanueva della Cañada, 28691, Spain
5  Geneva Observatory, University of Geneva, ch. des Maillettes 51, 1290 Sauverny, Switzerland
6  CEA/DSM/Dapnia, CEA-Saclay, 91191 Gif sur Yvette Cedex, France

Received 20 May 2008 / Accepted 19 September 2008

Aims. 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 0.15–1 TeV energies by the MAGIC telescope.
Methods. We analyse the hard X-ray emission from 18 to 700 keV 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.
Results. Despite variations in the flux by a factor of ~2 in the 20–700 keV 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 $\Gamma$ $\simeq$ 1.4 and a high energy cut-off at $E_{\rm c}$ $\simeq$ 130–140 keV. The spectrum is also well described by thermal Comptonisation including a moderate reflection component, with a solid angle of the reflector of ~ 0.4 $\times$ 2$\pi$. The temperature of the hot Comptonising electrons is $kT_{\rm e}$ ~ 70 keV and their Thomson optical depth is $\tau$ ~ 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 2 Crab, 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.

Key words: black hole physics -- accretion, accretion discs -- radiation mechanisms: non-thermal -- methods: observational -- stars: individual: Cyg X-1 -- X-rays: binaries

© ESO 2008