  We re-analysed the archival data of RXTE / INTEGRAL / Swift satellites at the
onset phase of the 2011 outburst of the X-ray source IGR J17091-3624. The
evolution of the spectral and temporal properties of the source in this phase
clearly exhibits state transition as Hard (HS) to Hard-Intermediate (HIMS) to
Soft-Intermediate (SIMS) to Soft (SS) state, before entering the variability
phase (VP).We attempt to understand the evolution of X-ray features and the
state transitions based on two different types of accreting material (i.e, the
Keplerian and sub-Keplerian flow).
