This paper presents the results of a dense and intensive X-ray and optical monitoring of the narrow-line Seyfert 1 galaxy NGC 4051 carried out in 2000 .
Results of the optical analysis are consistent with previous measurements .
The amplitude of optical emission line variability is a factor of two larger than that of the underlying optical continuum , but part or all of the difference can be due to host-galaxy starlight contamination or due to the lines being driven by the unseen UV continuum , which is more variable than the optical continuum .
We measured the lag between optical lines and continuum and found a lower , more accurate broad line region size of 3.0 \pm 1.5 light days in this object .
The implied black hole mass is M _ { BH } = 5 ^ { +6 } _ { -3 } \times 10 ^ { 5 } M _ { \odot } ; this is the lowest mass found , so far , for an active nucleus .
We find significant evidence for an X-ray–optical ( XO ) correlation with a peak lag \raisebox { -2.15 pt } { $ \stackrel { < } { \sim } $ } 1 day , although the centroid of the asymmetric correlation function reveals that part of the optical flux varies in advance of the X-ray flux by 2.4 \pm 1.0 days .
This complex XO correlation is explained as a possible combination of X-ray reprocessing and perturbations propagating from the outer ( optically emitting ) parts of the accretion disc into its inner ( X-ray emitting ) region .