IUE Observations of NGC 5548, 1978--1988: The Sizes and Kinematics of the Broad-Line Region

Abstract

We have reextracted the IUE spectra of NGC 5548 taken between 1978 and 1988 and studied changes in the UV continuum flux and UV line intensities. We have used the cross-correlation method advocated by Gaskell & Sparke and refined by Gaskell & Peterson to investigate the structure and kinematics of the broadline region (BLR). We give an extensive discussion of the errors in the cross-correlation method used and of the method's applicability to nonoptimum archival data. Using the cross-correlation analysis, we find that the total flux in the C IV λ1549 line gives a peak in the cross-correlation function (CCF) corresponding to an inner radius of 8 +/- 7 light-days, C III] λ1909 gives one at 11 +/- 6 light-days, Lyα λ1216 one at 5 +/- 5 light-days, Si IV/O IV] λ1400 one at 40 +/- 11 light- days, and He II λ1640 one at 5 +/- 6 light-days. Mg II λ2800 failed to give a significant peak in the CCF. We have also determined the luminosity-weighted radii for the different lines from the centroid of the CCFs. These radii are 31 light-days for C IV λ1549, 68 light-days for C III] λ1909, 4 light-days for Lyα λ1216, 60 light-days for Si IV/O IV] λ1400, and 71 light-days for He II λ1640. Monte Carlo simulations indicate that the probability of being misled into thinking that the peak in the CCF is 7 days or less when in reality it is 50 days is 0.16. Similar simulations for the centroid of the CCF show that the probability of being misled into thinking that the C IV size is 31 light-days or less if it is really over 100 light-days (say) is only a couple of percent. The differences in size for different lines are not statistically significant, but cross-correlation of the C III] and C IV lines suggests that the higher ionization line comes fro,m a smaller radius. Cross- correlation of the wings of the C IV line shows the blue wing following the red wing variations by 2 +/- 5 days (i.e., consistent with zero), implying predominantly chaotic or circular motion. Pure outflow and pure inflow are excluded at the 99% confidence level. We estimate the mass of the black hole to be about 2.2^+0.8^_0.9_ x 10^8^ M_sun_. These results are in excellent agreement with those obtained during just the 1989 observing season by the international NGC 5548 IUE consortium.