A spectroscopic and photometric study of short-timescale variability in NGC 5548

Results of a ground-based optical monitoring campaign on NGC 5548 in June 1998 are presented. The broad-band fluxes (U, B, V ), and the spectrophotometric optical continuum flux F (5100 A) monotonically decreased in flux while the broad-band R and I fluxes and the integrated emission-line fluxes of H and H remained constant to within 5%. On June 22, a short continuum flare was detected in the broad band fluxes. It had an amplitude of about18% and it lasted only90 min. The broad band fluxes and the optical continuum F (5100 A) appear to vary simultaneously with the EUV variations. No reliable delay was detected for the broad optical emission lines in response to the EUVE variations. Narrow H emission features predicted as a signature of an accretion disk were not detected during this campaign. However, there is marginal evidence for a faint feature at ' 4962 Aw ithFWHM' 6 Ar edshifted byv' 1100 km s 1 with respect to Hnarrow.