Black hole X-ray binary IGR J17091-3624 shows a unique intensity variation pattern

ATel #3667; Mayukh Pahari (TIFR), Sudip Bhattacharyya (TIFR), J S Yadav (TIFR)
on 30 Sep 2011; 15:18 UT
Credential Certification: Mayukh Pahari (mp@tifr.res.in)

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IGR J17091-3624 is a black hole X-ray binary which showed variability similar to the 'rho'-type variability seen in the lightcurve of GRS 1915+105 in two periods of ~26 days (started on 14 March 2011) and ~39 days (started on 12 July 2011) respectively as observed with RXTE (ATel #3230, #3266, #3418, see arXiv1105.4694 for details). Apart from that, from the RXTE data we have found very prominent dips with time durations of 20 s - 30 s (for example, see lightcurves on 04 September 11, 28 August 11 etc.), and these dips, considering the rising and the decay parts, are very similar to the dips seen in the 'omega' class (Pahari and Pal, 2010, MNRAS, 409, 903) or 'theta' class (Belloni et al., 2000, A&A, 355, 271) of GRS 1915+105. On 28th September 2011, the RXTE PCA lightcurve of this source showed a unique intensity variation: alternate appearance of (1) a roughly non-varying profile (average count rate is ~150 counts/s/PCU), and (2) a profile varying between ~50 counts/s/PCU and ~300 counts/s/PCU with an approximate period of 17s. We have created the corresponding hard color (ratio of count rate in 12.0-60.0 keV to that in 2.0-5.0 keV) profile, and found that there is a peak in hard color corresponding to an intensity dip, but there is no significant dip in hard color corresponding to an intensity peak. The latter is very unusual for a black hole X-ray binary, as these sources show significant spectral softening during a peak/flaring activity. To check the degree of hardness of the non-varying portions of the light curve, we have fitted the 2.5-20.0 keV spectra with a disk blackbody + powerlaw model modified with hydrogen column density. The column density has been found to be 3.68 +/- 0.66, which is ~3 times the value reported earlier (arXiv1105.4694). The best-fit disk inner edge temperature and power law index are 1.79 +/- 0.07 keV and 3.40 +/- 0.83 respectively. The unabsorbed power law flux is 70.3% of the total flux in the range of 2.0-60.0 keV. The reduced chisq of the fitting is 21.32/36 considering 0.1% systematic error. This confirms that the non- varying portions are spectrally hard. Between 0.01 Hz and 1 Hz in the power density spectra (PDS), the fractional rms amplitude the of the oscillatory portions of the light curve is at least 62 times higher than that of the non-varying portions. A very broad hump like structure is seen at 0.18 +/- 0.01 Hz in the PDS of the non-varying portions of the lightcurve. More RXTE observations and multi-wavelength studies are encouraged.