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S. Markoff, H. Falcke (Max Planck Institut für Radioastronomie, Bonn, Germany), R. Fender (Astrononomical Institute "Anton Pannekoek", University of Amsterdam, the Netherlands)
Observations reveal strong evidence for powerful jets in the Low/Hard states of black hole candidate X-ray binaries, and so it is important to consider what signatures these outflows will have in the broadband spectrum. Correlations between the radio and X-ray bands, and seemingly also with the near-infrared and optical, suggest that significant contributions to the observed broadband spectrum originate via synchrotron and possibly inverse Compton emission. We show here that, for reasonable assumptions about jet physical parameters, it is in fact hard to avoid dominant synchrotron emission from the radio through at least the infrared, and possibly X-rays as well. In the context of the recently discovered black hole binary XTE J1118+480, we explore a relatively simple model for relativistic, adiabatically expanding jets with accelerating shocks, combined with thermal disk emission. The entire data set up through X-ray can be well fit with a dominant jet contribution. In this case the X-ray power-law emission is direct, optically thin synchrotron, because the required low disk luminosity results in cooling time scales which allow acceleration to high particle energies. This model can also qualitatively explain observed time lags between different frequencies. We suggest that X-ray binaries in the Low/Hard state could be analogous to BL Lacs in the AGN field, which are also dominated by non-thermal jet emission. {\em S.M. is funded by a Humboldt Research Fellowship}.