Research Papers in Physics and Astronomy


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Published in Astronomy & Astrophysics 371, 79-92 (2001). DOI: 10.1051/0004-6361:20010330. Published by EDP Sciences; copyright ESO 2001. Online at


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 Fλ (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 Fλ (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 λ ≃ 4962 Å with FWHM ≃ 6 Å redshifted by Δv ≃ 1100 km s–1 with respect to Hβnarrow.

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