Evidence of two unique variability classes from IGR J 17091 – 3624

IGR J17091−3624 is the second black hole X-ray binary after GRS 1915+105, which showed large and distinct variabilities. The study of these variability classes can be useful to understand the accretion–ejection mechanisms of accreting black holes, and hence to probe the strong gravity regime. We report the discovery of two new variability classes (C1 and C2) from IGR J17091−3624 from the 2011 outburst Rossi X-ray Timing Explorer data. These unique classes will be useful to have complete details about the source, and to learn new aspects about variabilities. For example, the C1 class shows that the intensity and period of oscillations, energy spectrum and power spectrum can clearly evolve in tens of seconds. Moreover, in such a small time-scale, soft-lag becomes hard-lag. The C2 class shows that the variability and the non-variability can occur at similar energy spectrum, and a soft state is not required for variability to happen.