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

We interpret the rapid correlated ultraviolet/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 multiwavelength variability of XTE J1118+480. Assuming that the energy is stored in the form of a 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 time-scales for the X-ray emitting plasma is consistent with the dynamical times of an accretion flow between 10 and 100 Schwarzschild radii. On the other hand, the derived range of time-scales associated with the dissipation in the jet extends to time-scales 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(j) less than or similar to 3 x 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.