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Bright flare of Cyg X-1 in hard X-ray band

ATel #1533; A. Neronov (ISDC); M. Cadolle Bel (ESAC); S. Shaw (Southampton Uni.); J. Malzac (CESR); C. Sanchez (ESAC); P. Laurent, A. Goldwurm, S. Corbel (CEA-Saclay); S. Chaty (SAp); P. O. Petrucci (LAO-Grenoble)
on 19 May 2008; 16:49 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Simon Shaw (s.e.shaw@phys.soton.ac.uk)

Subjects: X-ray, Gamma Ray, Request for Observations

Referred to by ATel #: 1536

ESA's INTEGRAL/ISGRI instrument has detected a bright flare from the persistent black-hole binary Cyg X-1 during Key Program observations of the Cygnus region. During the first four days of the monitoring program the source flux was at a more or less constant level of about 1 Crab (20-80 keV). On top of this, INTEGRAL has detected a flare lasting about 8 hours and reaching a maximum of about 2 Crab in the 40-80 keV band in a pointing taken on April 18, 2008 between 15:02 and 15:36 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 with the photon index of 1.55+/-0.03, a high-energy cut-off at 161+/-13 keV and the 20-200 keV flux of 4.7e-8 erg/cm^2/s. The source was in the field of view of the X-ray monitor JEM-X during the flare. A factor of ~2 increase of the 3-10~keV flux from the source during the flare is observed, with a peak flux reaching the level of 0.54 Crab.

The flare is also visible in the RXTE/ASM lightcurve of the source.

The flux and spectral characteristics of the source in its brightest state are comparable to the historically brightest INTEGRAL state of the source detected in September 2006 (see ATEL #911), during which an evidence for the TeV gamma-ray emission from the source was found by MAGIC telescope (Albert et al. 2007, ApJL 665, 51).

The monitoring of the region with INTEGRAL will continue till April 23, 2008. We encourage observations of the source in the whole electromagnetic spectrum, especially in radio and VHE gamma-ray energy bands.

A plot of the lightcurve can be found here: