The variability classes in IGR J17091-3624 during the 2011 outburst

Aims. Based on the Rossi X-ray Timing Explorer (RXTE) data, we studied the typical variability classes in IGR J17091-3624 during the 2011 outburst and attempted to understand the physical mechanism that produces the highly structured variability. Methods. Using the rate-resolved spectroscopy, we produced color-color diagrams (CCDs), hardness-intensity diagrams (HIDs), and rate-resolved energy spectra for each class. The time lags were calculated by means of the cross-power spectrum (CPS) analysis. Results. Our study provides a comprehensive understanding of the nu, rho, alpha, beta/lambda, mu and gamma classes during this outburst. The source shows up with different track directions in the CCD loops and different types of the lag at similar to 1-6 s which are class dependent. It is notable that each variability class showing hard lags presents an anti-clockwise loop in the CCD, while each variability class showing soft lags presents a clockwise loop in the CCD. We discuss these observational results with the current models proposed for this type of system. The observed hardness ratio shows an increasing trend toward the high disk temperature for different classes, which can be regarded as evidence for thermal-viscous instability.