RXTE observes fast QPO evolution in IGR J17091-3624 indicating possible state transition

ATel #3179; Nikolai Shaposhnikov (UMD/CRESST/NASA Goddard)
on 22 Feb 2011; 06:10 UT
Credential Certification: Nikolai Shaposhnikov (nikolai.v.shaposhnikov@nasa.gov)

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Analysis of the last three RXTE observations of IGR J17091-3624 on Feb. 18 22:39 UT, Feb. 19 18:16 UT and Feb. 20 00:23 UT show fast evolution of the source variability properties. Namely, quasi-periodic oscillations (QPO) frequency, initially reported at 0.1 Hz on Feb 14 by Rodriguez et al. 2011, Atel #3168, is observed at 0.34, 0.45 and 1.66 Hz on Feb 18,19 and 20 correspondingly. In addition, during the last observation the second QPO harmonic is observed at ~3.3 Hz. A detailed spectral analysis is not possible due to contamination from a nearby source GX 349+2. However, the hardness (defined as a ratio of count rates in 9-20 keV and 4-9 keV energy ranges) during the three observation decreased from 0.164 on Feb. 18 to 0.156 on Feb. 19 and to 0.146 on Feb 20, indicating that the spectrum of IGR J17091-3624 is getting softer. This behavior is consistent with the source making a transition to the soft state. Taking the latest QPO values, it can reach soft intermediate state within a few days. This stage is associated with the outflow/jet activity in black hole candidates. Radio detection of the source by Corbel et al. (2011, Atel #3167) supports this expectation. Observations at other wavelengths are strongly encouraged.