Swift XRT and BAT observations of IGR J17091-3624 in the intermediate state
ATel #15295; Melania Del Santo, Alberto Segreto, Antonino D'Ai, Thomas Russell, Elena Ambrosi, Ciro Pinto, Fabio Pintore (INAF/IASF Palermo, Italy), Sara Motta (INAF/OAB, Italy), Alessio Marino (ICE-CSIC, Spain)
on 24 Mar 2022; 18:36 UT
Credential Certification: Melania Del Santo (melania@ifc.inaf.it)
Subjects: X-ray, Black Hole, Transient
Following the recent reactivation of the BH transient IGR J17091-3624 (ATEL #15282, #15283, #15284, #15286, #15287),
we triggered a monitoring campaign with the Neil Gehrels Swift Observatory (Swift).
Swift observed the source on 2022-03-19 (XRT exposure 1.3 ks) and on 2022-03-22 (XRT exposure 1.4 ks).
The 0.5-10 keV XRT spectrum in these two observations shows an increased count rate (from 58.5+/-0.2 ct/s to 78.2+/-0.2 ct/s) and spectral softening (2-10 keV photon index from 1.33 +/- 0.04 to 2.43 +/- 0.05), similarly to what reported in ATEL #15287.
We fitted simultaneously the BAT survey hard X-ray spectrum (15 keV up to 100 keV)
stacked from 2022-03-21 to 2022-03-23 with the XRT spectrum (0.5-10 keV) taken on March 22th
with a combination of an absorbed disc blackbody plus a thermal Comptonisation model.
The best fit results in an absorption column density of N_H=1.87+/-0.05 cm^-2, an inner disc temperature of 1.2 +/- 0.1 keV,
a power-law index of 1.7 +/- 0.4 and an electrons temperature of 7 +4/-2 keV.
Thus, the spectral parameters are consistent with IGR J17091-3624 being in the intermediate state.
The estimated fluxes in the 1-10 keV and 15-100 keV energy bands are 5E-9 erg/cm^2/s and 5E-10 erg/cm^2/s, respectively.
We extracted all the BAT spectra from the beginning with a time bin of 2 days and fitted them with a simple power-law model. The hard X-ray peak of the outburst occurred around MJD 59652 at a flux level of 1E-09 erg/cm2/s (in 15-50 keV). We found a significant evolution of the power-law index, which indicates a gradual hard to intermediate state transition (see link below).
Future Swift observations are planned in the upcoming weeks to monitor the outburst evolution. We thank the Swift team for assistance and quick planning of the observations.
BAT flux and spectral evolution