SPECTROSCOPIC MONITORING OF ACTIVE GALACTIC NUCLEI .3. SIZE OF THE BROAD-LINE REGION IN NGC-3227

We present in this work the results of a five-month monitoring campaign of the active galactic nucleus in NGC 3227. Both the Balmer Hbeta and Halpha emission lines, as well as the optical continuum have been analysed. A deblending procedure has been applied which allows to separate the contributions to the line emission from the narrow line region (NLR) on one side and from the broad line region (BLR) on the other side. The stellar contribution to the observed continuum within a 1.5 x 4 arcsec2 slit is found to be in the range 24-40% when the AGN is in the lowest state of activity observed during this monitoring period. The largest source of uncertainty in deriving the light-curves of the BLR emission and of the AGN continuum emission comes from differences in the seeing conditions prevailing at the various observational epochs. We have applied a correction factor for this effect. The optical AGN continuum and the BLR emission in NGC 3227 appear to vary by about 40% on a timescale of 1.5 month. We have followed a burst with maximum around early March and minimum by mid-April 1990. The levels of the broad line emission and the lambda6750 continuum are well correlated, a fact consistent with the classical assumption that the BLR material is photoionized by the central continuum source. Cross-correlation analyses between the light-curves of the lambda6750 AGN continuum and the BLR Halpha emission indicate that the lag of the BLR emission with respect to the continuum variations is 17 +/- 7 d. This result suggests that the BLR clouds which are affected by changes in the flux level of the central ionizing source lie about 17 light-days away. The variable part of the Halpha line shows a flat-topped profile with FWHM approximately 2900 km s-1. Within the observed range of continuum fluctuations (approximately 40%), the broad wings of the Halpha emission line do not vary in a noticeable way. This implies either that the BLR clouds building up the wings remain fully ionized during the continuum variations or that the time sampling should be increased if one wished to measure such an effect. Introduced together with other AGN in the plane {Balmer lag, luminosity}, NGC 3227 contributes to define a relationship slightly less steep than the BLR size scaling as the square root of the absolute luminosity.