Abstract We present the results of the spectral fits made to 59 Rossi X-ray Timing Explorer (RXTE) observations of the Galactic X-Ray Black-Hole Candidate XTE J1650–500 covering the first 30d of its 2001/2002 outburst when the source was in a transition from the hard state to the soft state. The photon spectra can be well fitted with a phenomenological model of a power-law/cutoff power-law and a physical model of bulk-motion Comptonization. The spectral properties smoothly evolve away from the hard state and then stay in the soft state. The fitting results of the physical model reveal the peak of the burst had a flux of 2.90 × 10−8 erg cm−2s−1 in the 2–100 keV energy range and was observed on 2001 Sep. 9; it transitioned to the hard state. The total flux decays by a factor of ~3 as it evolves into the soft state. The photon index Γ increases from ~1.5 in the hard state and stays at ~2.5 in the soft state. We found that the effective area of the high-energy X-ray emission region (the Compton cloud) decreases, i.e. the area of the Compton cloud decreases by a factor of ~23 during the transition from the hard state to the soft state. Combining the new radio and quasi-periodic oscillation studies, the model of total flux in the 2–100 keV energy range, the jet emission and the timing analysis during the state transition, we suggest a possible geometry and evolution for the (jet+corona+disk) system, like that proposed by Kalemci et al. based on enhanced lags and peak frequency shift during the transition.

Keywords accretion, accretion disks — black hole physics — X-rays binaries: in- dividual (XTE J1650–500) — X-rays: stars

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