  In this work, we study the X-ray timing and spectral evolution of the
transient low mass X-ray binary IGR J17091-3624 during first 66 days of its
2011 outburst. We present results obtained from observations with two
instruments, Rossi X-ray Timing Explorer (RXTE)/Proportional Counter Array
(PCA) and SWIFT/X-ray telescope (XRT), between 09 February, 2011 and 15 April,
2011. Using quasi-periodic oscillation classifications, power density spectrum
characteristics, time-lag behavior and energy spectral properties, we determine
source states and their transitions at different times of the outburst. During
the first part of the evolution, the source followed trends usually observed
from transient black hole X-ray binaries (BHXBs). Interestingly, a gradual
transition is observed in IGR J17091-3624 from the low-variability SIMS,
commonly seen in BHXBs, to a high-variability state with regular, repetitive
and structured pulsations, seen only from GRS 1915+105 (also known as 'rho'
class variability/'heartbeat' oscillations). We study the time evolution of
characteristic time-scale, quality factor and rms amplitude of heartbeat
oscillations in IGR J17091-3624. We also present a detailed comparison of the
timing and spectral properties of heartbeat oscillations and their evolution in
IGR J17091-3624 and GRS 1915+105.
