Black hole X-ray binaries ( BHXBs ) are known to display five distinct spectral states .
In order of increasing luminosity these are the quiescent state , low state , intermediate state , high state and very high state .
We present a self-consistent model of accretion flows around black holes which unifies all of these states except the very high state .
The model is an extension of the following paradigm which has been applied successfully to the quiescent state .
The accretion flow consists of two zones , an inner advection-dominated accretion flow ( ADAF ) which extends from the black hole horizon to a transition radius r _ { tr } , and an outer thin accretion disk that is present beyond r _ { tr } .
Above the disk is a hot corona which is a continuation of the inner ADAF .
The model consistently treats the dynamics of the accreting gas , the thermal balance of the ions and electrons in the two-temperature ADAF and corona , and the radiation processes that produce the observed spectrum .

At low mass accretion rates , { { { { \dot { m } \mathrel { \mathchoice { \vbox { \offinterlineskip \halign { \cr } $ \displaystyle% \hfil < $ \cr$ \displaystyle \hfil \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $% \textstyle \hfil < $ \cr$ \textstyle \hfil \sim$ } } } { \vbox { \offinterlineskip \halign { % \cr } $ \scriptstyle \hfil < $ \cr$ \scriptstyle \hfil \sim$ } } } { \vbox { \offinterlineskip% \halign { \cr } $ \scriptscriptstyle \hfil < $ \cr$ \scriptscriptstyle \hfil \sim$ } } } } 0.01 ( in Eddington units ) , the inner ADAF zone in the model radiates extremely inefficiently , and the outer thin disk is restricted to large radii ( r _ { tr } \sim 10 ^ { 2 } -10 ^ { 4 } , in Schwarzschild units ) .
The luminosity therefore is low , and this configuration is identified with the quiescent state .
For { { { { \dot { m } \mathrel { \mathchoice { \vbox { \offinterlineskip \halign { \cr } $ \displaystyle% \hfil > $ \cr$ \displaystyle \hfil \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $% \textstyle \hfil > $ \cr$ \textstyle \hfil \sim$ } } } { \vbox { \offinterlineskip \halign { % \cr } $ \scriptstyle \hfil > $ \cr$ \scriptstyle \hfil \sim$ } } } { \vbox { \offinterlineskip% \halign { \cr } $ \scriptscriptstyle \hfil > $ \cr$ \scriptscriptstyle \hfil \sim$ } } } } 0.01 and up to a critical value \dot { m } _ { crit } \sim 0.08 , the radiative efficiency of the ADAF increases rapidly and the system becomes fairly luminous .
The spectrum is very hard and peaks around 100 keV .
This is the low state .
The exact value of \dot { m } _ { crit } depends on the viscosity parameter \alpha ( \dot { m } _ { crit } \sim 1.3 \alpha ^ { 2 } ; the paper assumes \alpha = 0.25 ) .
For values of \dot { m } > \dot { m } _ { crit } and up to a second critical value about 10 % higher , the ADAF progressively shrinks in size , the transition radius decreases , and the X-ray spectrum changes continuously from hard to soft .
We identify this stage with the intermediate state .
Finally , when \dot { m } is sufficiently large , the inner ADAF zone disappears altogether and the thin accretion disk extends down to the marginally stable orbit .
The spectrum is dominated by an ultrasoft component with a weak hard tail .
This is the high state .
Model spectra calculated with this unified scenario agree well with observations of the quiescent , low , intermediate and high states .
Moreover , the model provides a natural explanation for the low state to high state transition in BHXBs .
We also make a tentative proposal for the very high state , but this aspect of the model is less secure .

A feature of the model is that it is essentially parameter-free .
We test the model against observations of the soft X-ray transient Nova Muscae during its 1991 outburst .
The model reproduces the observed lightcurves and spectra surprisingly well , and makes a number of predictions which can be tested by observations of other BHXBs .