The present paper describes the analysis of multiple RXTE /PCA data of the black hole binary with superluminal jet , XTE J 1550 - 564 , acquired during its 1999–2000 outburst .
The X-ray spectra show features typical of the high/soft spectral state , and can approximately be described by an optically thick disk spectrum plus a power-law tail .
Three distinct spectral regimes , named standard regime , anomalous regime , and apparently standard regime , have been found from the entire set of the observed spectra .
When the X-ray luminosity is well below \sim 6 \times 10 ^ { 38 } ~ { } { erg~ { } s ^ { -1 } } 
( assuming a distance of 5 kpc ) , XTE J 1550 - 564 resides in the standard regime , where the soft spectral component dominates the power-law component and the observed disk inner radius is kept constant .
When the luminosity exceeds the critical luminosity , the apparently standard regime is realized , where luminosity of the optically thick disk rises less steeply with the temperature , and the spectral shape is moderately distorted from that of the standard accretion disk .
In this regime , radial temperature gradient of the disk has been found to be flatter than that of the standard accretion disk .
The results of the apparently standard regime 
are suggestive of a slim disk ( e.g. , Abramowicz et al .
1988 , Watarai et al .
2000 ) which is a solution predicted under high mass accretion rate .
In the intermediate anomalous regime , the spectrum becomes much harder , and the disk inner radius derived using a simple disk model spectrum apparently varies significantly with time .
These properties can be explained as a result of significant thermal inverse Comptonization of the disk photons , as was found from GRO J 1655 - 40 in its anomalous regime by Kubota , Makishima and Ebisawa ( 2001 ) .