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The X-ray energy spectra and Normalized Power Spectral Densities (NPSDs) ofan X-ray nova, GS 2000+25, were investigated. The X-ray energy spectra of thesource consist of two components: a hard component, which can be represented bya power-law, and an ultra-soft component, represented by radiation from anoptically-thick accretion disk (the disk component). In a model in which thepower-law component is the Compton-scattered radiation, it is found that thetemperature of the incident blackbody radiation to the Compton cloud decreasefrom 0.8 keV to 0.2 keV according to the decay of the intensity, whichcoincides with that of the inner accretion disk. When the source changed fromthe high-state to the low-state, both the photon index of the power-lawcomponent (or Compton y-parameter) and the NPSD of the hard componentdramatically changed as did GS 1124-683. That is, the photon index changed from2.2--2.6 to 1.7--1.8 and the absolute values of the NPSDs at 0.3 Hz of the hardcomponent in the low-state became about 10-times larger than those of the hardcomponent in the high-state. These X-ray properties were similar to those ofother black-hole candidates, such as Cyg X-1, GX 339-4, and LMC X-3.Comment: 24 pages, text only, Full version is published in PASJ 54, 609-627  (2002

Energy Spectra and Normalized Power Spectral Densities of X-Ray Nova GS 2000+25