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The soft spectral state of the black hole candidate IGR J17091-3624 observed by INTEGRAL and Swift

ATel #3203; M. Del Santo (INAF/IASF-Roma, Italy), E. Kuulkers (ESA/ESAC, Spain), E. Bozzo (ISDC, Switzerland), F. Capitanio (INAF/IASF-Roma, Italy), J. Alfonso-Garzon (LAEX-CAB/INTA-CSIC, Spain), V. Beckmann (APC, France), T. Bird (Southampton, UK), S. Brandt (DTU Space, Denmark), J. Chenevez (DTU Space, Denmark), T. Courvoisier (ISDC, Switzerland), A. Domingo (LAEX-CAB/INTA-CSIC, Spain), K. Ebisawa (ISAS, Japan), P. Jonker (SRON, The Netherlands), P. Kretschmar (ESA/ESAC, Spain), C. Markwardt (GSFC, USA), T. Oosterbroek (ESA/ESTEC, The Netherlands), A. Paizis (INAF/IASF-Milano, Italy), K. Pottschmidt (UMBC/NASA GSFC, USA), C. Sanchez-Fernandez (ESA/ESAC, Spain), R. Wijnands (UvA, The Netherlands)
on 2 Mar 2011; 18:42 UT
Credential Certification: Melania Del Santo (melania.delsanto@iasf-roma.inaf.it)

Subjects: X-ray, Binary, Transient

Referred to by ATel #: 3225, 3229, 3230, 3266, 3299, 3913, 4773

The currently on-going outburst of the black hole candidate (BHC) IGR J17091-3624 (ATel #3144, #3159, #3167) has been recently observed simultaneously with INTEGRAL and Swift. The source was in the IBIS FOV on 2011 Feb. 28 from 17:45 to 21:23 (UTC; exposure time 7.7 ks) during the Galactic Bulge monitoring (Kuulkers et al. 2007, A&A, 466, 595) and pointed with the narrow field instrument on board Swift, XRT, on 2011 Feb. 28 from 19:00 to 20:36 (UTC; exposure time 1.2 ks).

Fitting the XRT spectrum with an absorbed power law model results in an unacceptable reduced chi^2=1.4 (304 d.o.f.). However, a better description of the spectrum (confirmed by the F-test) can be obtained adding a disk black-body component (red. chi^2=1.1 (302 d.o.f.)). The fit of the joint XRT+IBIS/ISGRI broad-band spectrum (0.8-200 keV) gives an absorption column density of N_H=1.00+/-0.06, a disc black-body temperature of 1.0+/-0.1 keV and a power-law photon index of 2.2+/-0.2 (red. chi^2=1.1 (312 d.o.f.)). No high-energy cut-off is requested up to 200 keV. The estimated fluxes in the 2-10 keV and 20-100 keV energy bands are 2E-09 erg/cm^2/s and 9E-10 erg/cm^2/s, respectively.

These results confirm that IGR J17091-3624 is presently in the canonical high/soft spectral state of BHCs (see also ATel #3179), unlike the previous reported INTEGRAL observation (Feb. 07-08) during which the source was in a low/hard state (ATel #3159).

Further simultaneous INTEGRAL and Swift observations are already planned.

We thank the Swift team for the rapid scheduling of simultaneous Swift ToO observations with the pre-planned INTEGRAL Galactic Bulge monitoring.