Toward Understanding the Spectral Energy Distribution of Microquasars

(Abridged) We report results from a systematic study of the spectral energy distribution (SED) of XTE J1550--564 and its evolution during outbursts. The main objectives include: 1) assessing the roles of jets and accretion flows; 2) re-examining spectral states, state transitions, and spectral hysteresis; and 3) disentangling variabilities associated with the jets and accretion flows and quantifying the coupling between the two. XTE J1550--564 is the first microquasar to have its jets directly imaged at X-ray energies. We began by applying a synchrotron model to the broadband SED of the western ``blob'' (in the jets) that was seen in 2002, to constrain the spectral energy distribution of radiating electrons in the ``blob''. Assuming that the electron distribution is the same for all ``blobs'' in the jets, we then applied the model to the overall radio spectrum of the source and extrapolated the best fits to higher frequencies. In spite of significant degeneracy in the fits, it seems clear that the synchrotron radiation from the jet contributes little to the observed X-ray emission, when the source is relatively bright. Assuming that the X-ray emission originated mostly from the accretion flows, we took an empirical approach to model the X-ray spectrum and study its evolution. From the compilation of the SEDs, we found it straightforward to define spectral states based on the shape of SEDs and visualize transitions between various states thus defined. There is evidence for spectral hysteresis associated with the rise and fall of the 2000 outburst. We can also see that a specific state transition may occur at different fluxes. We also examined correlation between the X-ray and radio fluxes of the source. While a rough correlation seems to be present, it is a fairly loose one. The implication of the results is discussed