Results of ongoing Swift/XRT monitoring of the low mass X-ray binary IGR J17091-3624

ATel #8834; Ralf Ballhausen (Remeis/FAU/ECAP), Victoria Grinberg (MIT), Joern Wilms (Remeis/FAU/ECAP), Felix Fuerst (CalTech), Wiebke Eikmann (Remeis/FAU/ECAP), Enrico Bozzo (ISDC), Marion Cadolle Bel (MPCDF), Elise Egron (INAF-OAC), Thierry Favre (ISDC), Carlo Ferrigno (ISDC), Felicia Krauss (Remeis/FAU/ECAP), Ingo Kreykenbohm (Remeis/FAU/ECAP), Michael A. Nowak (MIT), Katja Pottschmidt (CRESS/GSFC/UMBC), Jerome Rodriguez(AIM/CEA), Matteo Bachetti (INAF-OAC)
on 17 Mar 2016; 13:06 UT
Credential Certification: Joern Wilms (j.wilms@sternwarte.uni-erlangen.de)

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The low mass X-ray binary IGR J17091-3624 has been reported to be in outburst by Miller et al. (ATel #8742) on 2016 February 26. Subsequent observations by Swift/XRT and INTEGRAL revealed the transient to be in the hard/low state (Grinberg et al., ATel #8761). Photometry of the counterpart candidate using the GROND instrument at the ESO La Silla observatory and X-ray spectroscopy of further Swift/XRT observations were carried by Greiner et al. (ATel #8795). Radio observations with the SRT could not detect the source at the expected position (Egron et al., ATel #8821).

We report on the spectral evolution of IGR J17091-3624 as observed in a series of ~1 ks snapshot observations with Swift/XRT from 2016 March 1 through 2016 March 15. All observations were taken in WT mode and did not suffer from significant pile-up. We extracted spectra from source and background regions of 100" width, re-binned to minimum SNR of 7 and restricted to the energy range of 0.8--10 keV. We fitted all spectra simultaneously with an absorbed powerlaw model (similar to Grinberg et al., ATel #8761, Greiner et al., ATel #8795) with a global N_H for all observations (see Kuehnel et al. 2016, Acta Polytechnica 56, 41, for a description of the method).

All spectra can be well described with a global N_H of 1.429+0.013-0.012 e22 cm^-2 (90% CL). The unabsorbed flux in the 2--10 keV range increases from 0.29e-9 erg/s/cm^2 to 0.95e-9 erg/s/cm^2. During the observation on 2016 March 14, the source was very close to the gap in the CCD. This can introduce an unknown systematic uncertainty in the flux measurement. The photon index softens only slightly with flux with values ranging from 1.42+-0.08 to 1.63+-0.04. The parameter evolution over time is shown here. The source evolution is consistent with the source still being in the hard state, but slightly softening towards a possible transition to the intermediate state. Serendipitous INTEGRAL observations of the source during the Galactic Center monitoring program confirm this behavior. We caution that due to the Quick Look nature of the data, parameters may change slightly once the data are fully processed.

We strongly encourage a continuation of Swift/XRT monitoring and other multiwavelength monitoring to catch the transition to soft/intermediate state.

We thank the Swift PI and team for approving and scheduling the dense monitoring of IGR J17091-3624.