Correlated X-ray and optical variability in KV UMa

We present coordinated X-ray and optical observations of the X-ray transient KV UMa (= XTE J1118+480) at high time resolution. The optical variation associated with spikes in the X-rays consist of a dip preceding the spike by 0-5 s, and a peak following it. The cross-correlation function between X-rays and optical has the same shape. It rises sharply, within 30ms of the peak of the X-rays. The shape of the cross correlation shows significant intrinsic variability on time scales as short as 30s. Analyzing this variability with principal component analysis (calibrated with simulated data), we find two statistically independent components. The first is similar to the average cross-correlation. The second also consists of a dip followed by a peak, but on a 3x shorter time scale. These properties of the optical light, together with the high optical brightness of the transient, are not easily explained by reprocessing of the X-rays.