Magnetic flares and the optical variability of the X-ray transient XTE J1118+480

The simultaneous presence of a strong quasi-periodic oscillation, of period similar to 10 s, in the optical and X-ray light curves of the X-ray transi ent XTE J1118+480 suggests that a significant fraction of the optical flux originates from the inner part of the accretion flow, where most of the X-r ays are produced. We present a model of magnetic flares in an accretion dis c corona where thermal cyclo-synchrotron emission contributes significantly to the optical emission, while the X-rays are produced by inverse Compton scattering of the soft photons produced by dissipation in the underlying di sc and by the synchrotron process itself. Given the observational constrain ts, we estimate the values for the coronal temperature, optical depth and m agnetic field intensity, as well as the accretion rate for the source. With in our model we predict a correlation between optical and hard X-ray variab ility and an anticorrelation between optical and soft X-rays. We also expec t optical variability on flaring time-scales (similar to tens of ms), with a power-density spectrum similar to that observed in the X-ray band. Finall y, we use both the available optical/extreme-ultraviolet/X-ray spectral ene rgy distribution and the low-frequency time variability to discuss limits o n the inner radius of the optically thick disc.