Jet-disc coupling through a common energy reservoir in the black hole XTE J1118+480

MNRAS, submitted

J. Malzac (1), A. Merloni (2), A. C. Fabian (1)

1) Institute of Astronomy, Cambridge, CB3 0HA, UK
2)  Max-Planck-Institut fuer Astrophysik, K-Swarzschild Str., 1, 85741, Garching, Germany



We interpret the rapid correlated  UV/optical/ X-ray variability of XTE J1118+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 XTE J1118+480. 
Assuming that the energy is stored in the form of magnetic field, we
find that the required values of the  model parameters
are compatible with both a patchy corona atop a cold accretion disc and
a hot thick inner disc geometry.
The range of variability timescales for the X-ray emitting plasma are consistent with the dynamical times of an
accretion flow between 10 and 100 Schwarzschild radii. On the other hand,  the derived range of timescales associated with the
dissipation in the jet extends to timescales more than 10 times larger, confirming the suggestion that the generation of a powerful
outflow requires large scale coherent poloidal field  structures.
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, implying a radiative
efficiency for the jet $\epsilon_{\rm j} \la 3 \times 10^{-3}$.
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.