  We interpret the rapid correlated UV/optical/ X-ray variability of
XTEJ1118+480 as a signature of the coupling between the X-ray corona and a jet
emitting synchrotron radiation in the optical band.We propose a scenario in
which the jet and the X-ray corona are fed by the same energy reservoir where
large amounts of accretion power are stored before being channelled into either
the jet or the high energy radiation. This time dependent model reproduces the
main features of the rapid multi-wavelength variability of XTEJ1118+480. A
strong requirement of the model is that the total jet power should be at least
a few times larger than the observed X-ray luminosity. This would be consistent
with the overall low radiative efficiency of the source. We present independent
arguments showing that the jet probably dominates the energetic output of all
accreting black holes in the low-hard state.
