BeppoSAX Observations of the Power and Energy Spectral Evolution in the Black Hole Candidate XTE J1650-500

We study the time variability and spectral evolution of the black hole candidate source XTE J1650-500 using the BeppoSAX wide energy range (0.12-200 keV) observations performed during the 2001 X-ray outburst. The source evolves from a low/hard state (LHS) toward a high/soft state (HSS). In all states, the emergent photon spectrum is described by the sum of Comptonization and soft (disk) blackbody components. In the LHS, the Comptonization component dominates in the resulting spectrum. On the other hand, during the HSS observed by BeppoSAX the soft (disk) component is already dominant. In this state, the Comptonization part of the spectrum is much softer than that in the LHS (photon index Γ is ~2.4 in the HSS vs. Γ ~ 1.7 in the LHS). In the BeppoSAX data, we find a strong signature of the index saturation with the mass accretion rate, which can be considered as an observational evidence of the converging flow (black hole) in XTE J1650-500. We derive power spectra (PSs) of the source time variability in different spectral states as a function of energy band. When the source undergoes a transition to softer states, the PS as a whole is shifted to higher frequencies, which can be interpreted as a contraction of the Compton cloud during hard-soft spectral evolution. It is worthwhile to emphasize a detection of a strong low-frequency red noise component in the HSS PS, which can be considered a signature of the presence of the strong extended disk in the HSS. Also as a result of our data analysis, we find a very weak sign of K_α line appearance in this BeppoSAX data set. This finding does not confirm previous claims by Miniutti et al. on the presence of a broad and strongly relativistic iron emission line in this particular set of BeppoSAX data.