The Disappearing Broad Absorption Lines and Variable Emission Lines in NGC 3516

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Abstract

The Seyfert 1 galaxy NGC 3516 was monitored during 1993 February 16- May 13 by IUE every 4 days for the first month, and then every 2 days for 2 months giving a total of 40 observations. This paper gives the initial results from this campaign. (1) The broad C IV λ1549 emission line variations are delayed relative to those of the ionizing continuum by ~4.5 days, consistent with that of other similar luminosity active galactic nuclei. (2) The mean 1993 UV continuum level was about a factor of 3 higher than the mean archive spectrum and was at a record level throughout the campaign. (3) There was dramatic variability in the C IV λ1549 line profile when compared to the IUE archival data. A narrow absorption line is present in all of the 1993 spectra. In contrast, the broad variable trough was undetectable, whereas it is a very strong feature in all of the IUE archival spectra. The N V λ 1240 and Si IV λ1400 lines showed a similar behavior. A variable broad absorption line (VAL) model is required to explain the archival data, but it is unclear if a VAL model alone can explain the 1993 IUE data. Photoionization models suggest that the lack of line troughs in 1993 cannot be caused simply by increased ionization of the VAL gas. We propose two distinct models to explain the 1993 spectra: either the VAL gas has been dissipated, leaving a narrow absorption line presumably due to a more distant absorber, or an emission component, possibly a bicone, has greatly increased its contribution to the blue wing. Dissipation of the VAL gas implies a dynamic absorber, possibly the ionized surface layer of a molecular torus. An extra emission component implies that the broad line region properties in NGC 3516 may be radically different than has hitherto been proposed.

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