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
Credential Certification: Nami Mowlavi (Nami.Mowlavi@obs.unige.ch)
Subjects: X-ray, Gamma Ray, Binary, Black Hole
Referred to by ATel #: 1533
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.