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

作者：Malzac J^*; Lubinski P; Zdziarski A A; Bel M Cadolle; Tuerler M; Laurent P

来源：Astronomy & Astrophysics, 2008, 492(2): 527-534.

DOI：10.1051/0004-6361:200810227

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

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 similar to 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 similar or equal to 1.4 and a high energy cut-off at E(c) similar or equal to 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 similar to 0.4 x 2 pi. The temperature of the hot Comptonising electrons is kT(e) similar to 70 keV and their Thomson optical depth is tau similar to 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 similar 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.

• 出版日期2008-12
• 单位中国地震局