Astronomy and Astrophysics – Astronomy
Scientific paper
May 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...180.2002p&link_type=abstract
American Astronomical Society, 180th AAS Meeting, #20.02; Bulletin of the American Astronomical Society, Vol. 24, p.757
Astronomy and Astrophysics
Astronomy
Scientific paper
The broad emission lines in the spectra of active galaxies vary in response to changes in the continuum luminosity, but with a small time delay (on the order of days to weeks) which is taken to be attributable to light-travel time effects in the broad-line region (BLR). Emission-line variability thus affords a powerful tool for determining the structure and kinematics of the BLR because the particular form of the emission-line ``light echoes'' depends on the geometry of the BLR and its orientation relative to the observer. In practice, this ``reverberation mapping'' technique requires large amounts of high-quality data. As a consequence, it is only within the last few years that significant progress has been made in this area, principally on account of massive cooperative efforts by observers. Large monitoring consortia, such as the International AGN Watch and the Lovers of Active Galaxies (LAG), have employed ground-based and space-based telescopes, notably the International Ultraviolet Explorer, to study optical and ultraviolet continuum and emission-line variability in active galaxies such as NGC 5548. I will summarize the current state of affairs in this expanding new field. I will discuss the underlying assumptions of the reverberation method, and review the progress which has been made to date. I will address remaining ambiguities, discuss new questions which have arisen, and outline plans for further observational efforts. I gratefully acknowledge support of this research at Ohio State University by the National Science Foundation (AST-8915258) and NASA (NAG5-1366 and NAG5-1824).
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