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