Optical and X-ray monitoring of the NLS1 galaxy Ark 564
  The Narrow-line Seyfert galaxy Ark 564 was monitored between June 04, 2000 and June 24, 2000 from Skinakas Observatory. The B,V,R and I band light curves show night to night variations of 0.1 mag. Comparison with X-ray data from a simultaneous ASCA observation shows that the X-ray variations are more pronounced than in the optical band. Cross correlation analysis yields a time lag of ~ 0.8 days in the sense that the X-rays are leading the optical but with low statistical significance. We discuss possible implications of our results to current physical models of AGN. 
I. E. Papadakis, W. Brinkmann, H. Negoro, E. Detsis, I. Papamastorakis   and M. Gliozzi