Steps Toward Determination of the Size and Structure of the
Broad-Line Region in Active Galactic Nuclei:
XI. Intensive Monitoring of the Ultraviolet Spectrum of NGC 7469
Wanders, I., et al. 1997, ApJS, 113, 69
From 1996 June 10 to July 29, the International Ultraviolet Explorer monitored the Seyfert 1 galaxy NGC 7469 continuously in an attempt to measure time delays between the continuum and emission-line fluxes. From the time delays one can estimate the size of the region dominating the production of the UV emission lines in this source. We find the strong UV emission lines to respond to continuum variations with time delays of about 2.3--3.1 days for Lya, 2.7 days for C IV (lambda)1549, 1.9--2.4 days for N V (lambda)1240, 1.7--1.8 days for Si IV (lambda)1400, and 0.7--1.0 day for He II (lambda)1640. The most remarkable result, however, is the detection of apparent time delays between the different UV continuum bands. With respect to the UV continuum flux at 1315 Å, the flux at 1485 Å, 1740 Å, and 1825 Å lags with time delays of 0.21, 0.35, and 0.28 days, respectively. Determination of the significance of this detection is somewhat problematic since it depends on accurate estimation of the uncertainties in the lag measurements, which are difficult to assess. We attempt to estimate the uncertainties in the time delays through Monte Carlo simulations, and these yield estimates of 0.07 days for the 1 uncertainties in the inter-band continuum time delays. Possible explanations for the delays include the existence of a continuum-flux reprocessing region close to the central source and/or a contamination of the continuum flux with a very broad time-delayed emission feature such as the Balmer continuum or merged Fe II multiplets.