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

¹ Department of Astronomy, University of Florida, 211 Bryant Space Science Center, Gainesville, FL 32611-2055, USA
² Landessternwarte Heidelberg, Königstuhl, 69117 Heidelberg, Germany
³ University of Nebraska, Lincoln, Department of Physics and Astronomy, Lincoln, NE 68588-0111, USA
⁴ Sternberg Astronomical Institute, Universitetskij Prospect, 13, 119899 Moscow, Russia
⁵ Special Astrophysical Observatory, Russian Academy of Science, Nyzknij Arkhyz, Karachaj-Cherkess Republic, 369167, Russia
⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁷ Crimean Astrophysical Observatory, p/o Nauchny, 98409 Crimea, Ukraine
⁸ Isaac Newton Institute of Chile, Crimean Branch, Chile

Corresponding author: M. Dietrich, dietrich@astro.ufl.edu

Received: 17 November 2000
Accepted: 6 February 2001

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 (5100 Å) monotonically decreased in flux while the broad-band R and I fluxes and the integrated emission-line fluxes of Hα and Hβ 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 (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 Hβ 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 Åwith Åredshifted by km s^-1 with respect to H.