Spectral Class Transition of GRO J1655-40 during 2005 Outburst in the Light of TCAF Model

Galactic transient black hole candidates often show notable changes in their X-ray properties during outbursts. In this paper we study the timing and spectral properties of the transient X-ray binary GRO J1655-40 during its 2005 outburst using the Two Component Advective Flow (TCAF) model as a local model in XSPEC. We analyze the data of PCA onboard the Rossi X-ray Timing Explorer (RXTE) satellite from February to October 2005. Here we present a detailed insight in to the mass accretion mechanism and the characteristics of the accretion disk around the black hole candidate throughout the period of outburst. Based on the TCAF model fitting, parameters of the accretion flow e.g., the Keplerian rate or disk rate, sub-Keplerian rate or halo rate, shock location and the compression ratio or the shock strength are derived. On the basis of the evolution of the accretion rate ratio (ARR) or the ratio of the halo rate to the disk rate and considering the presence or absence of the Quasi-Periodic Oscillations (QPOs), we have categorized the entire outburst into four distinct spectral states: hard, hard-intermediate, soft-intermediate, and soft. It reveals that these distinct spectral states form a characteristic q-like pattern or hysteresis loop following the sequence: hard – hard-intermediate – soft-intermediate – soft – soft-intermediate – hard-intermediate – hard. We find that during the rising or declining phases, where the QPO frequencies are increasing or decreasing respectively, there is an associated rise in the value of the halo rate compared to the disk rate. We have also observed that the ARR reaches its maximum on the day of transition from the hard state to the hard-intermediate state during the rising phase, and on the day of transition from the hard-intermediate state to the hard state during the declining phase.