2.4. Emission-Line Variability
The broad emission lines in AGN spectra can vary both in flux and in profile. Over time scales of months and years, the changes can be very dramatic, but on shorter time scales they are more subtle. The first detection of emission-line variations was by Andrillat & Souffrin 2, based on photographic spectra of the Seyfert 1 galaxy NGC 3516. There were a few subsequent reports 83, 76, but these cases seemed to be widely regarded as "curiosities" that did not generate much follow-up work. The basic problem was that only very large changes could be detected photographically or with the intensified television-type scanners that were commonly used in AGN spectroscopy from the mid-1970s to mid-1980s. In the few cases where clear variations were detected, the changes were often dramatic; there were sometimes claims of Seyferts changing "type" as broad components of emission lines appeared or disappeared.
Pronounced variability of broad emission-line profiles was detected in the early 1980s by a number of investigators. Profile variations were originally thought to be due to excitation inhomogeneities; excitation pulses propagating through a broad-line region (BLR) with an ordered velocity field would produce features that could propagate across the profile with time. This concept led to the development of reverberation mapping, which is described in detail in Sec. 3. Peterson 61 reviews early work on emission-line variability. A useful way to isolate the variable part of an emission line is shown in Fig. 4. The upper panel shows a mean spectrum formed from 34 individual HST spectra of NGC 5548. The lower panel shows the "root-mean square" (rms) spectrum which is formed from the same data simply by computing the rms flux at each wavelength. Constant features, such as narrow emission lines and host-galaxy flux, do not appear in the rms spectrum.
Figure 4. The top panel shows the mean spectrum computed from 34 HST spectra of the variable Seyfert 1 galaxy NGC 5548 40. The lower panel shows the rms spectrum based on variations around this mean. The rms spectrum thus isolates the variable components of the spectrum. Fluxes are in units of 10-15 ergs s-1 cm-2 Å-1.