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

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.