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INTEGRAL observations of Cygnus X-3

ATel #2644; J. A. Tomsick (SSL/UC Berkeley), A. Tramacere (ISDC), C. Ferrigno (ISDC), S. Corbel (Univ. Paris Diderot & CEA Saclay), A. Bodaghee (SSL/UC Berkeley), S. Migliari (ESAC, Spain), G. Pooley (MRAO), K. Pottschmidt (CRESST/UMBC/GSFC), J. Rodriguez (CEA Saclay), and J. Wilms (Dr. Karl Remeis Observatory/ECAP)
on 27 May 2010; 17:13 UT
Credential Certification: John A. Tomsick (jtomsick@ssl.berkeley.edu)

Subjects: X-ray, Gamma Ray, Binary, Black Hole

Referred to by ATel #: 2646, 3139, 3325, 8591

Evidence that the HMXB and microquasar Cygnus X-3 has been transitioning to its soft state has been reported since early-May (2010) in ATELs #2609, #2611, and #2635 using all-sky monitors Swift/BAT, RXTE/ASM, and MAXI. This is significant because it would represent the first soft state for Cyg X-3 since the reports that gamma-ray (>100 MeV) flaring seen by Fermi and AGILE is associated with this state.

Cyg X-3 was in the FOV of the IBIS/ISGRI telescope on-board INTEGRAL from 2010 May 24 (MJD 55340) at 14:50 to 2010 May 25 at 21:26 (UTC). During this period (effective exposure time 64 ks), the ISGRI spectrum is well-described (reduced chi^2 of 1.04 for 7 degrees of freedom) as a power-law with a photon index of 2.7+\-0.1 and an estimated flux of 3.9E-10 ergs/cm^2/s (20-40 keV).

During the same observational period, the source was in the FOV of JEM-X2 for a total effective exposure time of 1.4 ks. The JEM-X2 spectrum was well fit with an absorbed black-body model and gave N_H=(8+\-5)E22 cm^(-2) and kT=1.0+\-0.1 keV. The estimated flux was 6.0E-9 erg/cm^2/s in the energy band 4-10 keV.

While the shape of the X-ray spectrum and the fluxes measured with ISGRI and JEM-X2 confirm that Cyg X-3 is not in its hard state, the source has been seen in even softer states in the past. Our analysis of INTEGRAL data covering Cyg X-3 over the past few years includes the soft X-ray/gamma-ray flaring state from 2008 November. During that episode, the 20-40 keV flux measured with ISGRI for Cyg X-3 reached levels as low as 5.3e-11 ergs/cm^2/s averaged over a full INTEGRAL orbit (>2 days). Thus, while it is currently unclear whether >100 MeV gamma-ray flares have begun (see ATELs #2609 and #2611) for this state, it is possible that this will not occur until Cyg X-3 reaches an even softer X-ray state.

We thank the ISDC for providing the results of their quick look analysis.