Modelling the broad-band spectral energy distribution of the microquasars XTE J1550-564 and H 1743-322

We report results from a systematic study of the spectral energy distribution (SED) and spectral evolution of XTE J1550-564 and H 1743-322 in outbursts. The jets of both sources have been directly imaged at both radio and X-ray frequencies, which makes it possible to constrain the spectrum of the radiating electrons in the jets. We modelled the observed SEDs of the jet 'blobs' with synchrotron emission alone and with synchrotron emission plus inverse Compton scattering. The results favour a pure synchrotron origin of the observed jet emission. Moreover, we found evidence that the shape of the electron spectral distribution is similar for all jet 'blobs' seen. Assuming that this is the case for the jet as a whole, we then applied the synchrotron model to the radio spectrum of the total emission and extrapolated the results to higher frequencies. In spite of significant degeneracy in the fits, it seems clear that, while the synchrotron radiation from the jets can account for nearly 100 per cent of the measured radio fluxes, it contributes little to the observed X-ray emission, when the source is relatively bright. In this case, the X-ray emission is most likely dominated by emission from the accretion flows. When the source becomes fainter, however, the jet emission becomes more important, even dominant, at X-ray energies. We also examined the spectral properties of the sources during outbursts and the correlation between the observed radio and X-ray variabilities. The implication of the results is discussed.