We study hard states of the black-hole binary XTE J1550–564 during its 2000 outburst .
In order to explain those states at their highest luminosities , L \sim 10 \% of the Eddington luminosity , L _ { E } , we propose a specific hot accretion flow model .
We point out that the highest values of the hard-state L are substantially above the L an advection-dominated accretion flow ( ADAF ) can produce , \sim 0.4 \alpha ^ { 2 } L _ { E } , which is only \sim ( 3 – 4 ) \%L _ { E } even for \alpha as high as 0.3 .
On the other hand , we successfully explain the hard states with L \sim ( 4 – 10 ) \% using the luminous hot accretion flow ( LHAF ) model .
As 10 \%L _ { E } is also roughly the highest luminosity an LHAF can produce , such an agreement between the predicted and observed highest luminosities provides by itself strong support for this model .
Then , we study multi-waveband spectral variability during the 2000 outburst .
In addition to the primary maxima in the optical light curves , secondary maxima were detected after the transition from the very high state to the hard state .
We show that the secondary maxima are well modeled by synchrotron emission from a jet formed during the state transition .
We argue that the absence of the corresponding secondary peak in the X-ray light curve indicates that the X-ray jet emission , regardless of its radiative process , synchrotron or its Comptonization , is not important in the hard state compared to the emission from the accretion flow .