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 (firstname.lastname@example.org)
Subjects: X-ray, Binary, Transient
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.