Space telescope RM project. I. NGC5548

Significant correlated variability is found in the continuum and broad emission lines, with amplitudes ranging from ~30% to a factor of two in the emission lines and a factor of three in the continuum.

The variations of all the strong emission lines lag behind those of the continuum, with HeII{lambda}1640 lagging behind the continuum by ~2.5 days and Ly{alpha}{lambda}1215, CIV{lambda}1550, and SiIV{lambda}1400 lagging by ~5-6 days. The relationship between the continuum and emission lines is complex. In particular, during the second half of the campaign, all emission-line lags increased by a factor of 1.3-2 and differences appear in the detailed structure of the continuum and emission-line light curves.