IGR J17091-3624 shows a unique quasi-'rho' class variability during recent RXTE/PCA observations

ATel #3418; Mayukh Pahari (TIFR), J S Yadav (TIFR), Sudip Bhattacharyya (TIFR)
on 9 Jun 2011; 17:45 UT
Credential Certification: Mayukh Pahari (mp@tifr.res.in)

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Recent RXTE/PCA observations of the black hole low mass X-ray binary IGR J17091-3624 showed a unique quasi-'rho' class variability. This variability consisted of oscillations with very large amplitude and oscillations with small amplitude. Both oscillations occurred alternately for a certain interval of time. Interestingly, both oscillations have same timescale and seen first time in this source. Comparing with the variability seen in lightcurves of GRS 1915+105, this quasi-'rho' class of IGR J17091-3624 might be an intermediate between 'rho' class and 'mu' class of GRS 1915+105 (see Belloni et al. 2000, A&A, 355, 271 for definition of classes). Previously, the source showed prolonged (~ 26 days) 'rho'-like characteristic activities (ATel #3230, #3266, Pahari et al., arXiv:1105.4694v1) after which it showed variability similar to the 'beta' class of GRS 1915+105 (ATel #3299) for a short time. Then it entered a long steady period of 34 days (from 21 April 2011 to 23 May 2011). On 24 May 2011, it again showed 'rho'-like variability in lightcurves. This variability is very similar to that seen on 31 March 2011 from this source. The characteristic time scale is ~40.25 sec. From 26 May 2011 to 03 June 2011, the lightcurve showed a mixture of large bursts where peak-to-dip count rate ratio is ~3-5 and short bursts where peak-to-dip count rate ratio is ~1.5-2 . In these lightcurves, intervals of large bursts and short bursts appeared alternatively. Number of large burst intervals in a given time as well as the peak counts of large bursts decreased gradually from 26 May 2011 to 03 June 2011 and it disappeared on 04 Jun 2011. From 24 May 2011 to 28 May 2011, quasi-periodic oscillation (QPO) frequency corresponding to the large bursts in the power density spectrum increased from ~24.4 mHz to ~46.8 mHz. From 29 May 2011, the peak frequency started decreasing and decreased upto ~12.3 mHz on 05 June 2011. From 04 June 2011 to 06 June 2011, the lightcurve rarely showed any large burst and consisted of oscillations very similar to 31 March 2011 with the timescale of ~65-70 sec. From 23 May 2011 to 06 June 2011, the average count rate increased from ~85 cts/s/PCU to ~112 cts/s/PCU. It is interesting to note that, the maximum frequency of the fundamental QPO was found to be ~46.8 mHz which is same as that found on 10 April 2011 during previous 'rho'-like activity of the source (see Fig. 9 of Pahari et al., arXiv:1105.4694v1). This fact strengthens the limit on the timescale of disk activity during 'rho' class. Thus multi-wavelength and continuous monitoring of this source is encouraged.