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In a recent outburst which lasted for 260 days, the black hole candidate GROJ1655-40 exhibited a behaviour similar to its last outburst observed almosteight years ago. We analyze a total of 150 observational spells in 122 days ofdata spreaded over the entire outburst phase of Feb. 2005 to Oct. 2005. Fromour study, a comprehensive understanding of the detailed behaviour of thisblack hole candidate has emerged. Based on the degree of importance of theblack body and the power-law components we divide the entire episode in fourspectral states, namely, hard, soft, very soft and intermediate. Quasi-Periodicoscillations (QPOs) were found in two out of these four states, namely, in thehard and the intermediate states. In the hard state, at the rising phase of theoutburst, QPO frequency ranged from 0.034 - 17.78Hz and the spectra was fittedby a disk black body, power-law and iron emission line at 6.2 - 6.5 keV. In theintermediate state, QPOs vary from 13.17Hz to 19.04Hz and the QPO frequencymodulation in this state was not significant. The spectra in this state arewell fitted by the disk black body and the power-law components. In the hardstate of the declining phase of the outburst, we found QPOs of decreasingfrequency from 13.14 Hz to 0.034 Hz. The spectra of this state were fitted by adisk black body and power-law components, but in the initial few days a coolerComptonized component was required for a better fit. In the soft and very softstates, the spectral states are mostly dominated by the strong disk black bodycomponent.

Spectral and timing evolution of GRO J1655-40 during its outburst of 2005