3.3 Recent Developments
3.3.1 The NGG 5548 and 4151 Campaigns
The aim of the NGC 5548 campaigns (using IUE and ground-based optical observations) was to monitor the variability with an unprecedented sampling rate, in both the continuum and the lines. A previous and similar study was devoted to NGC 4151, from which Ulrich et al. 98 concluded that the continuum variability in UV and optical was simultaneous, with an upper time limit for the delay 2 days. The timescale of the variability was also of this order, which may have ruled out an origin due to local thermal disk instabilities. There was also a significant correlation between variations in UV-optical and X-rays on at least one occasion. Some of the NGC 5548 results 99-101 can be summarized as follows:
I. Ionization structure
The delay between continuum and line variations was smaller for highly ionized elements, and the line variability amplitude increased with the degree of ionization. This reinforces the belief in the photoionization hypothesis, and shows that the BLR is stratified.
Krolik 19, 102 and collaborators argued that detailed model-fitting implies that the HIL are emitted from a spherical region, whereas the LIL should be emitted from a flattened geometry, such as an accretion disk. That the HIL and LIL do not arise in the same region is indicated by their different profiles and redshifts. 103 The customary one-zone BLR model is thus not applicable.
Gaskell 85 used the cross-correlation technique on the wings of the CIV line in NGC 4151, and argued that radial inflow of the BLR clouds was indicated. The same arguments were applied to Fairall 9 (F9) 104 and NGC 5548. 105 In contrast, Clavel et al. 99 found no evidence of radical motion in the latter source, and argued in favour of chaotic or rotational motion. According to Ulrich, 72 the fast moving clouds should not cross the accretion disk, as in the case of chaotic motion. The clouds emitting the HIL should then remain on the same side of the disk after formation, and other forces (e.g., radiative or magnetic) besides gravitation may influence the cloud motion. The temporal behaviour of the CIV line wings in NGC 4151 also indicates that the spatial distribution of the clouds changes on a timescale ~ years, which may signal the arrival of new material from the inner BLR. 98
The CIV line is frequently used in BLR size estimates. The peak value of the cross-correlation between this line and the continuum is ~ 8 days, whereas the centroid amounts to ~ 31 days, which approximately correspond to the inner BLR radius and an ``average'', luminosity-weighted one, respectively. 106 Both of these values are consistent with those found in the campaigns, and the inferred BLR size (~ 0.01 pc) is about one order of magnitude lower than the standard value. Moreover, the stratification implies a distribution of clouds with different sizes, temperatures and distances from the continuum source. The concept of a single BLR ``radius'' may thus be replaced by a range, amounting to 10-100 light-days. 97 A size smaller than the standard value has also been found in F9, Mrk 279, NGC 4151 and Akn 120. 89