A Spectroscopic and Photometric Study of Short-Timescale Variability in NGC 5548

Abstract

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 wavelength (5100 Å) monotonically decreased in flux while the broad-band R and I fluxes and the integrated emission-line fluxes of H alpha and H beta remained constant to within 5% . On June 22, a short continuum flare was detected in the broad band fluxes. It had an amplitude of about ~18% and it lasted only ~ 90 min. The broad band fluxes and the optical continuum F wavelength (5100 Å) 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 beta 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 wavelength ~ 4962Åwith FWHM ~ 6 Åredshifted by delta v 1100kms-1 km s-1 with respect to H beta (narrow).