Modeling the Hard States of XTE J1550-564 during Its 2000 Outburst

We study the 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 ~ 10% of the Eddington luminosity, LE, 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 that an advection-dominated accretion flow can produce, ~0.4a2LE, which is only ~3%-4% of LE even for a as high as 0.3. On the other hand, we successfully explain the hard states with L ~ 0.04-0.10 using the luminous hot accretion flow (LHAF) model. As 0.10LE is also roughly the highest luminosity an LHAF can produce, such agreement between the predicted and observed highest luminosities itself provides strong support for this model. We then study multiwaveband 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 as synchrotron emission from a jet formed during the state transition. We argue that the absence of a corresponding secondary peak in the X-ray light curve indicates that the X-ray jet emission, regardless of its radiative process, synchrotron or Comptonization, is not important in the hard state as compared with the emission from the accretion flow.