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Paper text:
accreting black holes , both stellar ( including the galactic @xmath2quasars ) and supermassive , show strong , soft , thermal components in their euv  x - ray spectra , when accreting above certain fraction of eddington rate .
this component is thought to come from an optically thick accretion disk , and its spectrum is usually modeled as a simple , multi - color blackbody . for re - examining a number of such spectra , i will use the spectral models described in @xcite .
an example of is is the may 18th observation of gs  1124 - 68 ( nova muscae 1991 ) .
using first the frequently employed but unphysical model , diskbb + smedge(powerlaw ) , gives an unacceptable fit , @xmath3 .
adding a gaussian line at 6.4 kev does _ not _ improve the fit , unless the line width is @xmath4 kev ( @xmath5 ) . allowing the line energy to be free
gives @xmath6 for @xmath7kev .
these values have little to do with fe spectral features , but rather indicate that it is the continuum shape that is not properly modeled .
using now the diskbb + thcomp + rel - repr model gives very bad fit , @xmath8 .
replacing the diskbb component with a comptonized blackbody ( comptt ) results in a dramatic improvement , @xmath9 .
thus the best fit is obtained for the proper physical description of x  ray reprocessing and the _ comptonized blackbody _
model for the soft component .
the same effect is observed in gro j1655 - 40 , gs  2000 + 25 and xte j1550 - 564 ( ycki et al .
, in preparation ) .
nova muscae 1991 was in the high state when observed on mar 18th .
the spectrum shows a strong soft component , and the hard component is negligible @xcite .
modeling the spectrum with the diskbb + thcomp model gives @xmath10 ( see fig .
[ zycki_tiii-06:fig1 ] ) .
the comptonized component is steep and makes a significant contribution to the soft component .
again , the soft component has to be described by a model _ broader _ than the multi - color disk blackbody .      on jan 11th , while still on the rise to the peak of the outburst , nova muscae 1991 , showed a spectrum consisting of two comptonized components : the softer one again _ can not _
be described by the diskbb model .
it can be modeled as comptonization of @xmath11 kev blackbody by a warm , @xmath12 kev , optically thick , @xmath13 , plasma .
the additional comptonization seems to be a common feature of disk - dominated spectral states of accreting black holes .
there are two possible scenarios to explain it .
the first one is comptonization in a `` hot skin '' on top of the accretion disk .
such a hot layer could not be heated by x 
ray irradiation ( as postulated for the low / hard state ) , but it would require enhanced viscous energy dissipation compared to the bulk of the disk .
the second possibility is that the electron energy distribution in the comptonizing plasma is a hybrid , thermal  non - thermal one , rather than a pure thermal ( maxwellian ) distribution .
in fact , the compps model ( j. poutanen , private communication ) using the hybrid distribution gives very good fits to the hs and is data sets : @xmath14 for is , @xmath15 for hs , but it fails for vhs .
shortly before reaching the peak of outburst nova muscae 1991 showed flip - flop transitions @xcite : the flux in the 220 kev band changed by @xmath16 .
spectral and timing analysis during this period suggests that the source were undergoing re - structuring of the inner accretion disk .
energy spectra both in the high- and low - flux state can be adequately described by the diskbb + thcomp + rel - repr models ( no additional comptonization of the soft component is required ) .
the best - fit model spectra differ most at energies 510 kev : the differences are small both at the lowest and highest energies ( fig .  [ zycki_tiii-06:fig2 ] ) .
the difference spectrum can be described as comptonization of single blackbody emission , but it can not be described as comptonized _ disk _ blackbody .
x  ray power density spectra show both energy and flux dependence . the soft x
 rays ( @xmath17 kev ) pds is very different in the two flux states ( fig .
[ zycki_tiii-06:fig2 ] ) . in the high flux state
the pds resembles that of typical hs , i.e.  a power law @xmath18 with @xmath19 , although there is also a narrow qpo at @xmath20 hz .
however , in the low flux state the psd has a lorenzian shape peaking ( in the @xmath21 representation ) at @xmath22 hz .
the pds of the hard x  ray component ( @xmath23 kev ) is similar in both flux states except for the qpo energy which is higher in the low flux state .
interactions between accretion disk and corona in the soft spectral states of accreting black holes are clearly complex .
one outcome of those interactions is a presence of warm , mildly optically thick plasma modifying the disk thermal emission .
proper modeling of the resulting spectra is important e.g.  for correct determination of system parameters .
the flip - flop transitions observed in vhs of gs  1124 - 68 close to the peak of its outburst suggest that the optically thick accretion flow can proceed in two modes giving the same time - integrated characteristics .
one mode , usually observed in hs , shows the @xmath24-like pds , i.e.  a scale - invariant behaviour , perhaps realized in magnetic turbulence @xcite .
the other mode clearly possesses certain spatial / temporal scale , since the pds is described by a lorenzian , i.e.  it corresponds to exponential shots of a single time scale .
re - analysis of some archival x  ray data of black hole binaries in soft spectral states is presented . 
 results of detailed spectral fitting , including proper physical description of the x  ray reprocessed component , indicate that in most cases the soft , thermal disk emission can not be described by a simple multi - color blackbody model . 
 additional comptonization of those seed photons provides an acceptable description of the soft component , implying a presence of a warm ( @xmath0 kev ) , mildly optically thick ( @xmath1 ) phase of accreting plasma .    
 combined spectral / temporal analysis of gs  1124 - 68 during flip - flop transitions in vhs suggests that the optically thick accretion flow can proceed in ( at least ) two modes giving the same time - integrated behaviour ( i.e.  energy spectra ) but very different timing behaviour .