  We describe the first results from a six-month long reverberation-mapping
experiment in the ultraviolet based on 170 observations of the Seyfert 1 galaxy
NGC 5548 with the Cosmic Origins Spectrograph on the Hubble Space Telescope.
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 He II 1640 lagging
behind the continuum by ~2.5 days and Lyman alpha 1215, C IV 1550, and Si IV
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.
Velocity-resolved cross-correlation analysis shows coherent structure in lag
versus line-of-sight velocity for the emission lines; the high-velocity wings
of C IV respond to continuum variations more rapidly than the line core,
probably indicating higher velocity BLR clouds at smaller distances from the
central engine. The velocity-dependent response of Lyman alpha, however, is
more complex and will require further analysis.
