The Variability and Spectrum of NGC 5548 in the Extreme Ultraviolet

We have measured the light curve and spectrum of NGC 5548 obtained with the Extreme Ultraviolet Explorer (EUVE) during a period when the galaxy was also intensively monitored with the Hubble Space Telescope, the International Ultraviolet Explorer, and ground-based telescopes. NGC 5548 was observed with EUVE for a total of about 20 days spread over a period of 2 months. The broadband extreme ultraviolet (EUV) light curve showed several factor of 2 variations on 0.5 day timescales and one factor of 4 decrease over 2 days. The normalized variability index was smaller than in the EUVE observation of Mrk 478. Cross-correlation of the EUV light curve with the contemporaneous ultraviolet light curve indicates that the EUV and UV/optical variations are, to within the limitations of these data, simultaneous, although the amplitude in the EUV is twice that in the UV. The shape of the EUVE spectrum is consistent with a gradual steepening from the UV through the soft X-rays. Contrary to reports by others, we do not detect any emission lines in the EUV spectrum that would arise in optically thin spectral models. We believe that the reported line detections were, in fact, due to a low-level fixed pattern in the detector background. Models of the spectrum may be constrained by the time-averaged flux in the broadband detector combined with the weakly constrained spectral shape. A thermal model best describes the EUVE spectrum and its variations

Steps toward Determination of the Size and Structure of the Broad-Line Region in Active Galactic Nuclei. IX. Ultraviolet Observations of Fairall 9

An 8 month monitoring campaign on the Seyfert 1 galaxy Fairall 9 has been conducted with the International Ultraviolet Explorer in an attempt to obtain reliable estimates of continuum-continuum and continuumÈemission-line delays for a high-luminosity active galactic nucleus (AGN). While the results of this campaign are more ambiguous than those of previous monitoring campaigns on lower luminosity sources, we Ðnd general agreement with the earlier results : (1) there is no measurable lag between ultraviolet continuum bands, and (2) the measured emission-line time lags are very short. It is especially notable that the Lyα + N V emission-line lag is about 1 order of magnitude smaller than determined from a previous campaign by Clavel, Wamsteker, & Glass (1989) when Fairall 9 was in a more luminous state. In other well-monitored sources, speciÐcally NGC 5548 and NGC 3783, the highest ionization lines are found to respond to continuum variations more rapidly than the lower ionization lines, which suggests a radially ionization-stratified broad-line region. In this case, the results are less certain, since none of the emission-line lags are very well determined. The best-determined emission line lag is Lyα + N V, for which we find that the centroid of the continuum—emission-line crosscorrelation function is tcent ≈14—20 days. We measure a lag tcent<~4 days for He II λ1640; this result is consistent with the ionization-stratification pattern seen in lower luminosity sources, but the relatively large uncertainties in the emission-line lags measured here cannot rule out similar lags for Lyα + N V and He II λ1640 at a high level of significance. We are unable to determine a reliable lag for C IV λ1550, but we note that the profiles of the variable parts of Lyα and C IV λ1550 are not the same, which does not support the hypothesis that the strongest variations in these two lines arise in the same region

Steps foward determination of the size and structure of the broad-line region in active galactic nuclei. V. Variability of the ultraviolet continuum and emission lines of NGC 3783

We report on the results of intensive ultraviolet spectral monitoring of the Seyfert 1 galaxy NGC 3783. The nucleus of NGC 3783 was observed with the International Ultraviolet Explorer satellite on a regular basis for a total of 7 months, once every 4 days for the first 172 days and once every other day for the final 50 days. Significant variability was observed in both continuum and emission-line ftuxes. The light curves for the con­ tinuum ftuxes exhibited two well-defined local mínima or "dips," the first lasting <~20 days and the second <~4 days, with additional episodes of relatively rapid ftickering of approximately the same amplitude. As in the case of NGC 5548 (the only other Seyfert galaxy that has been the subject of such an intensive, sustained monitoring effort), the largest continuum variations were seen at the shortest wavelengths, so that the continuum became "harder" when brighter. The variations in the continuum occurred simultaneously at ali wavelengths (Δt < 2 days). Generally, the amplitude of variability of the emission lines was lower than (or comparable to) that of the continuum. Apart from Mg II (which varied little) and N v (which is relatively weak and badly blended with Lyα), the light curves of the emission lines are very similar to the continuum light curves, in each case with a small systematic delay or "lag." As for NGC 5548, the highest ionization lines seem to respond with shorter lags than the lower ionization lines. The lags found for NGC 3783 are consider­ ably shorter than those obtained for NGC 5548, with values of (formally) ~O days for He II +O III], and ~4 days for Lyα and C IV. The data further suggest lags of~4 days for Si IV+ O IV] and 8-30 days for Si III] +C III]. Mg II lagged the 1460 Å continuum by ~9 days, although this result depends on the method of measuring the line flux and may in fact be due to variability of the underlying Fe n lines. Correlation analysis further shows that the power density spectrum contains substantial unresolved power over timescales of <~2 days, and that the character of the continuum variability may change with time

Steps foward determination of the size and structure of the broad-line region in active galactic nuclei. V. Variability of the ultraviolet continuum and emission lines of NGC 3783

We report on the results of intensive ultraviolet spectral monitoring of the Seyfert 1 galaxy NGC 3783. The nucleus of NGC 3783 was observed with the International Ultraviolet Explorer satellite on a regular basis for a total of 7 months, once every 4 days for the first 172 days and once every other day for the final 50 days. Significant variability was observed in both continuum and emission-line ftuxes. The light curves for the con­ tinuum ftuxes exhibited two well-defined local mínima or "dips," the first lasting <~20 days and the second <~4 days, with additional episodes of relatively rapid ftickering of approximately the same amplitude. As in the case of NGC 5548 (the only other Seyfert galaxy that has been the subject of such an intensive, sustained monitoring effort), the largest continuum variations were seen at the shortest wavelengths, so that the continuum became "harder" when brighter. The variations in the continuum occurred simultaneously at ali wavelengths (Δt < 2 days). Generally, the amplitude of variability of the emission lines was lower than (or comparable to) that of the continuum. Apart from Mg II (which varied little) and N v (which is relatively weak and badly blended with Lyα), the light curves of the emission lines are very similar to the continuum light curves, in each case with a small systematic delay or "lag." As for NGC 5548, the highest ionization lines seem to respond with shorter lags than the lower ionization lines. The lags found for NGC 3783 are consider­ ably shorter than those obtained for NGC 5548, with values of (formally) ~O days for He II +O III], and ~4 days for Lyα and C IV. The data further suggest lags of~4 days for Si IV+ O IV] and 8-30 days for Si III] +C III]. Mg II lagged the 1460 Å continuum by ~9 days, although this result depends on the method of measuring the line flux and may in fact be due to variability of the underlying Fe n lines. Correlation analysis further shows that the power density spectrum contains substantial unresolved power over timescales of <~2 days, and that the character of the continuum variability may change with time