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\title{Jets in XRB Low/Hard States:  Models of XTE J1118+480 suggest a
strong signature from radio through X-ray\thanks{We are grateful to
the  (late)
Bob Hjellming for sharing his data with us.}} 
\author{S. \surname{Markoff}\thanks{Supported by a Humboldt
Research Fellowship}}
\author{H. \surname{Falcke}}
\institute{Max-Planck-Institut f\"ur Radioastronomie, Bonn, Germany}  
\author{R. \surname{Fender}}
\institute{Astronomical Institute ``Anton Pannekoek'', University of
Amsterdam, Netherlands}
\runningauthor{Markoff, Falcke \& Fender}
\runningtitle{Jets in XRBs}
\keywords{X-ray binaries --jets -- black hole physics -- radiation
mechanisms:non-thermal}
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\section*{Summary}
Observations reveal the presence of powerful jets in the Low/Hard
states of several black hole candidate X-ray binaries, so it is
important to consider what signatures these outflows will have in the
broadband Low/Hard spectrum. Correlations between the radio and
X-rays, as well as in at least one case the optical, suggest that
emission from a jet, via either synchrotron or inverse Compton (IC)
processes, may play a role.  The recently discovered X-ray transient
XTE J1118+480 (Remillard et al. 2000), a black hole candidate in the
Low/Hard X-ray state, provides an excellent opportunity for
multi-wavelength modeling, since it has been observed not only in the
radio through X-rays (see Fender et al. 2000, and references therein),
but is also at high enough Galactic latitude to allow the first ever
EUV detections of an X-ray transient (Hynes at al. 2000, hereafter
H00).

We consider a simplistic scenario of symmetric, magnetized
relativistic plasma outflows.  The jet plasma encounters a shock,
which accelerates the particle distribution into a power-law, creating
a jump in the spectrum where the shock-accelerated leptons produce an
optically thin power-law extending to the X-ray regime.  The details
of this calculation, as well as a more evolved model which addresses
the presence of an accretion disk component, can be found in Falcke \&
Markoff (2000), Markoff, Falcke \& Fender (2000), respectively, and references
therein.  We include a representative figure, which includes only
synchrotron emission from the jet.  Surprisingly, we can account for
the radio-IR and the X-ray data reasonably well without invoking an IC
component or the presence of significant disk emission.

As an interesting note, the hard X-rays have been seen lagging the
soft in several XRBs (e.g., Ford et al. 2000).  A multi-component
model such as that presented here, or including disk emission (see
Markoff et al. 2000) is favorable.

\begin{figure}
\psfig{figure=proc_fig.ps,width=4.5in,angle=0}
\caption{Fit for power-law spectral index 2.6, $M_{\rm bh}=5M_\odot$,
$L=0.04L_{\rm Edd}$, $z_{\rm sh}=16 r_s$, and inclination angle
40$^\circ$.  Data points from H00 except radio (Hjellming et al., in
prep.), and JCMT (Fender et al. 2000).  We cannot account for the
entire optical range from the jet alone, but the detection of strong
$H_\alpha$ lines (Garcia et al. 2000) could be indicative of disk
emission at these frequencies.  We use the maximum absorption case
presented in H00 for the EUVE data.  Included is a schematic
explaining the different emission regions.  The JCMT point obviously
is well outside the range of this model, which could be accounted for
either by the addition of another shock further out, resulting in
enhanced emission, or by thermal emission from a torus.}
\end{figure}

\begin{thebibliography}{}
\bibitem[2000]{FalckeMarkoff2000} Falcke, H. \& Markoff, S., 2000,
A\&A, in press
\bibitem[2000b]{Fenderetal2000} Fender, R.P., Hjellming, R.M.,
Tilanus, R.J., Pooley, G.G., Deane, J.R., Ogley, R.N., Spencer, R.E.,
2000, MNRAS, submitted
\bibitem[2000]{Fordetal2000} Ford, E.C., van der Klis, M., M{\'e}ndez,
M., Wijnands, R., Homan, J., Jonker, P.G., van Paradijs, J., 2000,
ApJ, 537, 368
\bibitem[2000]{Garciaetal2000} Garcia, M., Brown, W., Pahre, M.,
McClintock, J., Callanan, P., Garnavich, P., 2000, IAU Circ. 7392
\bibitem[2000]{Hynesetal2000} Hynes, R.I., Mauche, C.W., Haswell,
C.A., Shrader, C.R., Cui, W., Chaty, S., 2000, ApJL, 539, L37
\bibitem[2000]{MarkoffFalckeFender} Markoff, S., Falcke, H., Fender,
R., 2000, ApJL, submitted
\bibitem[2000]{Remillardetal2000} Remillard, R., Morgen, E., Smith, D.,
Smith, E., 2000, IAU Circ. 7389 
\end{thebibliography}
\end{article}
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