6 Summary

For the first time, theoretical thermal X-ray spectra were obtained for the dynamical parameters of a relativistic jet calculated from the MHD wind equation. The total spectra were derived as composition of the spectral contributions of the single volume elements accelerating along the jet with relativistic speed. Our results are the following.

1.

We find X-ray emission from the hot inner part of the jet originating in a region of $2.5 \times 10^{-5}$AU diameter close to the center of a 5 ${M}_{\odot}$ jet source. The jet X-ray luminosity is $L_{\rm X} \sim 10^{33}~(\dot{M}_{\rm j}/10^{-8}\,{M}_{\odot}\,{\rm yr}^{-1})\,{\rm erg\,s^{-1}}$.

2.

Emission lines of Fe XXV and Fe XXVI are clearly visible in our spectra. Interestingly, the $K\!\alpha$ iron emission line has been probably observed in GRS 1915+105 (Ebisawa et al. 1998) and XTE J1748-288 (Kotani et al. 2000). The absence of broad Fe-lines in the spectrum of SS433 might tell us something on the "invisibility'' of the acceleration region above the disk. Comparison of our calculated emission lines to observed ones may give some hints on the plasma composition in relativistic jets.

3.

From the MHD jet underlying the spectra we find a maximum Doppler boosting of about 7. Minimum boosting is present along the opposite side of the jet cone (Doppler factor 0.53). The shift of the emission lines is always visible. The boosting, however, does not play a major role in the total spectra, because of the uncollimated geometry of the innermost part of the jet emitting the X-rays and the combined effect of boosting and de-boosting around the jet cone.

If jets from X-ray binaries indeed contain matter of baryonic composition, our model will have a broad application. Indication of that is probably given by the observation of iron emisson lines in some sources (see above). However, it is not yet clear, whether the line emission originates in the jet or in the accretion disk. Our calculated Fe emission lines may help to interpret the observed spectra and potentially give some clue on the plasma composition in relativistic jets.

This study will be extended in a future work investigating spectra of jets with different magnetic geometry, mass flow rates and central masses. In the end, this might also allow to constrain the intrinsic parameters of jet formation itself (such as mass loading or opening angle) from the observation of the large-scale, asymptotic jet.

Acknowledgements

This work was partly supported by the German Science Foundation (Deutsche Forschungsgemeinschaft) as project DFG/FE490. We thank an anonymous referee for useful comments.