Multiple evolving peaks in the power density spectrum and the spectral transition were detected in IGR J17091-3624

ATel #3266; Mayukh Pahari (TIFR), Sudip Bhattacharyya (TIFR), J S Yadav (TIFR)
on 7 Apr 2011; 20:19 UT
Credential Certification: Mayukh Pahari (mp@tifr.res.in)

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The recent RXTE/PCA observations of the heartbeat oscillation class (HOC)(ATel #3230) in the black hole X-ray binary source IGR J17091-3624 (ATel #3144, #3159, #3167, #3203) showed a variable and highly evolved power density spectrum (PDS) with multiple peaks. Analyzing the PCA data, we found that this class (HOC), which showed similar lightcurve with the 'rho' class of GRS 1915+105 (ATel #3230) appeared to evolve with time and last longer than the usual 'rho' class. For example, the 10 mHz characteristic oscillations on 14th March, 2011 (ATel #3225) gradually changed to dual peaks at 0.022 +/- 0.002 Hz and 0.044 +/- 0.003 Hz on 29th March, 2011. Again, on 31st March, 2011 three prominent peaks were observed in the PDS. the details of each of them is furnished below:

Peak number	Peak centre (Hz)	Peak FWHM (Hz)	Fractional rms (%)	Q-value
1	0.023 +/- 0.003	0.0014 +/- 0.0002	19.35 +/- 5.32	6.43 +/- 3.24
2	0.051 +/- 0.001	0.0057 +/- 0.0018	7.95 +/- 2.54	8.95 +/- 2.23
3	0.079 +/- 0.002	0.013 +/- 0.004	6.89 +/- 2.11	6.07 +/- 1.91

On 3rd April, 2011 only two peaks were appeared in the PDS: 0.038 +/- 0.001 Hz and 0.073 +/- 0.002 Hz. Thus the 10mHz Quasi Periodic Oscillation (QPOs) frequency evolved significantly in a period of 20 days. Such evolution of PDS has not been seen in the 'rho' class of GRS 1915+105. The peak-shifting pattern in QPOs has also been found in observations of this source prior to the HOC class. From 9th February, 2011 to 10th March, 2011 the source appeared to be in the high state where 0.105 +/- 0.003 Hz oscillation (ATel #3168) on 10th February, 2011 shifted to 3.84 +/- 0.07 Hz on 2nd March, 2011. Although, the energy spectra of both heartbeat oscillation class as well as high state class fitted well (between 2.5-30.0 keV) with a combination of diskbb+powerlaw+Gaussian (with the chisq/dof of 29.94/50 and 28.87/50 respectively), the parameter values are different. In a gradual transition from the high state to the HOC, the disk temperature decreased from 2.55 +/- 0.08 keV (23rd February, 2011) to 1.19 +/- 0.04 keV (31st March, 2011) and disk normalization increased from 4.56 +/- 0.49 to 23.65 +/- 3.91 (similar values for the high state were reported for 2003 outburst by Capitanio et al., 2006, ApJ, 643, 376). the power spectral index did not vary much (~ 2.45 +/- 0.03) but the power-law normalization decreased from 2.11 +/- 0.14 to 0.29 +/- 0.03. Interestingly, absorption column density (in the unit of 10^(-22) cm^-2) is found to be quite high in high state (2.93 +/- 0.56) while it is low at the HOC (0.94 +/- 0.05). In the high state, the unabsorbed flux between 2.0-100.0 keV is found to be dominated by the power-law component (~79%) while the 2.0-100.0 keV unabsorbed flux in the HOC class was dominated by the disk blackbody component (~65 %). Hence, state transition is likely to occur between these two classes. Continuous monitoring of the source is encouraged.