Simultaneous Multiband Observation of Rapid Varibility in NGC 4051

Searche for rapid variability in the flux from active galaxies has the Promise of being a very powerful tool for understanding their emission mechanisms. Despite the keen Observational interest in the short characteristic time predicted, reports of rapid X-ray variability in active galaxies have been rare. One of them is NGC 4051 which has a characteristic time scale of-1000s.For confirming and extending these observation of NGC 4051, On May 13-14, 1985, we had observed simultaneously in the different places and different waveband, X-ray band by Martin Elvis and Chen Fuzhcn with EXOSAT in U. S. A., U Band by Xie Guangzhong et al. With photometry in Yunnan Observatory China. The main results are as follows: (1) The soft X-ray flux had decreased by a factor of thirteen, and hard X-ray were not detected (down by at least a factor of 203). (2) The U band results show that NGC 4051 has a very short characteristic time scale of approximatly 1200 s for optical variability with a amplitude of ΔU≈0.^m11 (Fig. 1). The curves of optical variability are perfect and each of the four significant events had larger than 3σ confidence, where a is the observation rms error which is equal to 0,^m03.