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 campaig n of the active galactic nucleus in NGC 3227. Both the Balmer Hbeta an d Halpha emission lines, as well as the optical continuum have been an alysed. A deblending procedure has been applied which allows to separa te 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 si de. 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 t he lowest state of activity observed during this monitoring period. Th e largest source of uncertainty in deriving the light-curves of the BL R 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 ar ound early March and minimum by mid-April 1990. The levels of the broa d line emission and the lambda6750 continuum are well correlated, a fa ct consistent with the classical assumption that the BLR material is p hotoionized by the central continuum source. Cross-correlation analyse s between the light-curves of the lambda6750 AGN continuum and the BLR Halpha emission indicate that the lag of the BLR emission with respec t 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 pa rt of the Halpha line shows a flat-topped profile with FWHM approximat ely 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 bui lding up the wings remain fully ionized during the continuum variation s or that the time sampling should be increased if one wished to measu re such an effect. Introduced together with other AGN in the plane {Ba lmer lag, luminosity}, NGC 3227 contributes to define a relationship s lightly less steep than the BLR size scaling as the square root of the absolute luminosity.