2.5. The First Monitoring Programs
The early 1980s saw the first attempts to monitor the UV/optical continuum and emission-line variations in Seyfert 1 galaxies. There were two reasons this happened when it did. First, there was a realization that variability afforded a powerful probe of the structure and kinematics of AGNs on projected scales of microarcseconds. Variability was recognized as an important new tool with which to study enigmatic quasars. Second, the right technology for such investigations became widely available: it was possible to attempt such programs on account of (a) IUE, which for the first time allowed precision UV spectroscopy of low-redshift extragalactic objects, and (b) the proliferation of linear electronic detectors (first Reticons and Image Dissector Scanners, and later CCDs) on moderate-size (1-2m) ground-based telescopes.
One of the first significant monitoring programs was a multiple-year IUE-based program on NGC 4151, which was carried out by a European consortium led by M.V. Penston and M.-H. Ulrich 84. Ultraviolet spectra were obtained with a typical sampling interval of 2-3 months. The program showed that variations in the UV and optical continua were closely coupled. It also revealed that the emission-line flux variations are correlated with continuum variations, but that different lines respond in different ways, both in amplitude and in time scale. These data also showed a complicated relationship between UV and X-ray variations and led to the discovery of variable absorption lines in the ultraviolet.
The galaxy NGC 4151 was also monitored spectroscopically in the optical at Lick Observatory by Antonucci & Cohen 4. They found that the Balmer lines seemed to respond to continuum variations on a time scale less than around one month (their typical sampling interval). They also found that relative to H and H, the higher-order Balmer lines and He II 4686 varied with higher amplitudes.
Arakelian (Akn) 120 was the first higher-luminosity Seyfert that was monitored fairly extensively in the optical 64 , 65 as a result of dramatic Balmer-line profile changes that had been detected earlier 26 , 38, 80 . Peterson et al.cite 65 found that the time scale for the response of H to continuum variations suggested a BLR size of less than 1 light month across. This was a surprising result as it suggested that there was a serious problem with existing estimates of sizes of the BLR that were based on photoionization equilibrium modeling, as these indicated the BLR should be about an order-of-magnitude larger than this. The upper limit on the BLR size was similar to that obtained by Antonucci & Cohen 4 for NGC 4151, but because Akn 120 is a higher-luminosity source, the monthly sampled data provided a more critical challenge to BLR models.
Not surprisingly, the results from these earlier monitoring programs were controversial. Several observational problems could be identified:
Correlated errors are due to systematic flux-calibration errors. Basically, if the flux calibration of a spectrum is incorrect, both the continuum and emission-line fluxes measured from it will be in error in the same sense; if the calibration is too high, both the emission-line and continuum fluxes will be too high. This introduces an artificial correlation between the line and continuum at zero lag, and can thus bias the measurement of the true lag between them to artificially small values.
Aperture effects occur when the amount of flux entering a spectrograph is not fixed, on account of pointing or guiding errors, or variations in seeing in the case of ground-based observations. In point-like sources like stars, this affects only the overall photometric accuracy. In nearby AGNs, however, both the narrow-line region (NLR) and host-galaxy are spatially resolved, and the aperture geometry, centering and guiding, and seeing variations can lead to apparent spectral variations. Depending on their nature, aperture effects can cause either correlated, uncorrelated, or even anticorrelated errors in the line and continuum fluxes.