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 1 998 are presented. The broad-band fluxes (U, B, V), and the spectrophotomet ric optical continuum flux F lambda (5100 Angstrom) monotonically decreased in flux while the broad-band R and I fluxes and the integrated emission-li ne fluxes of H alpha and H beta remained constant to within 5%. On June 22, a short continuum are was detected in the broad band fluxes. It had an amp litude of about similar to 18% and it lasted only approximate to 90 min. Th e broad band fluxes and the optical continuum F lambda (5100 Angstrom) appe ar to vary simultaneously with the EUV variations. No reliable delay was de tected for the broad optical emission lines in response to the EUVE variati ons. Narrow H beta emission features predicted as a signature of an accreti on disk were not detected during this campaign. However, there is marginal evidence for a faint feature at lambda similar or equal to 4962 Angstrom wi th FWHM similar or equal to 6 Angstrom redshifted by Delta upsilon similar or equal to 1100 km s(-1) with respect to H beta (narrow).