ULTRACAM observations of the black hole X - ray transient XTE J 1118 + 480 in quiescence

We present high time-resolution multicolour observations of the quiescent soft X-ray transient XTEJ1118+480 obtained with ULTRACAM. Superimposed on the double-humped continuum g ′ and i -band lightcurves are rapid flare events which typically last a few minutes. The power density spectrum of the lightcurves can be described by a broken power–law model with a break frequency at ∼2mHz or a power– law model plus a broad quasi-periodic oscillation (QPO) at ∼2mHz. In the context of the cellular-automaton we estimate the size of the quiescent advection-dominated flow (ADAF) region to be ∼ 10 Schwarzschild radii, similar to that observed in other quiescent black hole X-ray transients, suggesting the same underlying physics. The similarites between the low/hard and quiescent state PDS suggest a similar origin for the optical and X-ray variability, most likely from regions at/near the ADAF.