Swift Monitoring Suggests IGR J17091-3624 is Transitioning to the Intermediate State

ATel #8858; J. M.C. Court (Uni. Southampton), S. E. Motta (Uni. Oxford), D. Altamirano (Uni. Southampton)
on 23 Mar 2016; 15:50 UT
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Credential Certification: Sara Elisa Motta (sara.motta@physics.ox.ac.uk)

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The black hole candidate low mass X-Ray Binary IGR J17091-3624 was reported to re-enter outburst on 26th February 2016 by Miller et al. (ATel #8742) based on a steady rise seen in the Swift/BAT light curve. The outburst started approximately on February 20th (based on Swift/BAT, 15-50keV) and the count rate increased to a maximum of 0.020 +/- 0.002 cnt/s on March 14 (see also ATel #8834, Swift/BAT lightcurves of the outburst can be found here). Subsequent Swift/BAT measurements show a significant drop in 15-50keV count rate to 0.015 +/- 0.003 cnt/s on March 22. Swift/XRT spectra taken before March 22 indicate that, up to that time, the source had remained in the hard state (ATel #8761, #8834).

Swift/XRT observed IGR J17091-3624 on March 22nd, for a total exposure of 1077 seconds. We extracted the Swift/XRT spectrum in a region centered at the source position with radius equal to ~0.8 arcmin. The background corrected count rate is 23.1 +/- 0.2 cnt/s. We modelled the XRT spectrum in the 0.8-10.0 keV energy band using absorbed (equivalent column density = (1.53 +/- 0.04) x 10^22 atoms/cm^2) Comptonized emission from neutral material (Comps in XSPEC), assuming that the seed photons are coming from a (cool) disk-blackbody spectrum. Since the seed photon temperature is not constrained, we froze it to 0.1keV. No disk-blackbody component nor a reflected component are statistically required to model the spectrum. Our best fit gives a derived photon index of 2.43 +/- 0.04. The absorbed and unabsorbed fluxes in the 2-10keV are 1.04e-09 ergs/cm^2/s and 1.15e-09 ergs/cm^2/s respectively, slightly brighter than the value of 0.95e-09 ergs/cm^2/s reported on March 15 (Atel #8834).

Swift/XRT energy spectrum of March 22 is significantly softer than the spectrum of March 16, for which the best-fit photon index was reported as 1.64 +/- 0.4 (Atel #8834). All the above suggests that IGR J17091+3624 might be transitioning into an intermediate or even soft state.

About 18 days after the transition to the soft state during the 2011 outburst, IGR J17091-3624 started to show GRS 1915+105-like 'heartbeat' X-ray variability (e.g., Altamirano et al. 2011, ApJL, 742, 17). Assuming the 2016 outburst will show a behaviour similar to that seen in 2011, we expect to see similar phenomenology starting by early to mid-April.

Further Swift/XRT are needed to confirm the transition to the intermediate/soft state. Swift will continue monitoring the source during the upcoming days. Multi-wavelength monitoring is strongly encouraged, especially high time and spectral resolution X-ray observations of IGR J17091-3624 throughout March and April 2016 to observe possible 'heartbeat' variability.