Spectral and timing evolution of GRO J1655-40 during its outburst of
  2005
  In a recent outburst which lasted for 260 days, the black hole candidate GRO
J1655-40 exhibited a behaviour similar to its last outburst observed almost
eight years ago. We analyze a total of 150 observational spells in 122 days of
data spreaded over the entire outburst phase of Feb. 2005 to Oct. 2005. From
our study, a comprehensive understanding of the detailed behaviour of this
black hole candidate has emerged. Based on the degree of importance of the
black body and the power-law components we divide the entire episode in four
spectral states, namely, hard, soft, very soft and intermediate. Quasi-Periodic
oscillations (QPOs) were found in two out of these four states, namely, in the
hard and the intermediate states. In the hard state, at the rising phase of the
outburst, QPO frequency ranged from 0.034 - 17.78Hz and the spectra was fitted
by a disk black body, power-law and iron emission line at 6.2 - 6.5 keV. In the
intermediate state, QPOs vary from 13.17Hz to 19.04Hz and the QPO frequency
modulation in this state was not significant. The spectra in this state are
well fitted by the disk black body and the power-law components. In the hard
state of the declining phase of the outburst, we found QPOs of decreasing
frequency from 13.14 Hz to 0.034 Hz. The spectra of this state were fitted by a
disk black body and power-law components, but in the initial few days a cooler
Comptonized component was required for a better fit. In the soft and very soft
states, the spectral states are mostly dominated by the strong disk black body
component.
