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INTEGRAL observation of Cyg X-1 at a historic hard X-ray maximum

ATEL # 911; M. Türler (ISDC, Geneva), A.A. Zdziarski (Copernicus A.C., Warsaw), P. Laurent (CEA, Saclay), L. Foschini (IASF, Bologna), J.-P. Roques (CESR, Toulouse), N. Lund (DNSC, Copenhagen), B. McBreen (Univ. College, Dublin), P. Kretschmar (ESA/ESAC, Madrid), N. Gehrels (NASA/GSFC), M. Revnivtsev (IKI, Moscow & MPA, Garching)
on 5 Oct 2006; 16:11 UT
Password Certification: Nami Mowlavi (

Subjects: X-ray, Gamma Ray, Binaries, Black Holes

ESA's INTEGRAL mission is regularly observing the persistent black-hole binary Cyg X-1 during monitoring observations of the galactic plane. On 24-26 September 2006 the source was at its highest hard X-ray level since INTEGRAL's launch in October 2002. The two-day long light-curve of this observation shows a more or less constant flux at a level of about 1.5 Crab (20-40 keV) and 1.8 Crab (40-80 keV). On top of this high state, INTEGRAL observes an outburst lasting about 8 hours and reaching a maximum of about 2.0 Crab (20-40 keV) and 2.3 Crab (40-80 keV) in a pointing taken on 25 September 2006 between 20:58 and 21:57 UT.

The spectrum obtained by the IBIS/ISGRI instrument in the 20-300 keV band during this pointing is well fitted by a cut-off power-law assuming 2% of systematic errors. The best fit parameters are a photon index of 1.61, a high-energy cut-off at 135 keV and a normalization at 1 keV of 4.49 ph/cm^2/s/keV. The integrated flux is 21.8 keV/cm^2/s in the 20-100 keV band and 9.8 keV/cm^2/s in the 100-300 keV band. This is among the highest daily fluxes observed by CGRO/BATSE from 1991 to 2000 in the harder band and exceeds all of them in the 20-100 keV band (Zdziarski et al. 2002, ApJ 578, 357).

As Cyg X-1 was always outside the JEM-X field-of-view we do not have simultaneous observations at lower X-ray energy. Further INTEGRAL observations of Cyg X-1 taken on 30 September 2006 indicate that the source returned to a more usual state at a level of about 1 Crab in the 20-40 keV band and 1.3 Crab in the 40-80 keV band.

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R. E. Rutledge , Editor
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