Correlation between the infrared and optical variations of NGC 4151 in 30 years: Change in the sizes of the infrared source

The correlation between infrared and optical nuclear variability of the Sey fert galaxy NGC 4151 is analyzed by using partly published photometric moni toring data for the last 30 years. A cross-correlation analysis is performe d both for the entire 30-year-long series and separately for the two states of high nuclear activity in 1969-1980 (cycle A) and in 1990-1998 (cycle B) . The lag between the infrared variability (the K band, 2.2 mu m) and the o ptical variability (the U band, 0.36 mu m) in cycle B was found to be Delta t = 35 +/- 8 days, which is approximately twice that in cycle A (Delta t = 18 +/- 6 days). The increase in lag correlates with an increase in optical luminosity. The lag for cycle B was found to depend on wavelength: Delta t = 6 and 8 days in J (1.25 mu m) and H (1.6 mu m), respectively, while in L (3.5 mu m) Delta t = 97 days, which is almost a factor of 3 larger than th at in K. The K and L emission is interpreted as thermal dust reradiation of the ultraviolet emission from a region close to the central source. Numeri cal simulations lead us to conclude that an emission-line region in the sha pe of a thin ring (torus) seen nearly face-on is in better agreement with o bservational data than the model of a spherical layer or a torus with a lar ge inclination to the line of sight. The effective torus radius depends on wavelength, which accounts for the larger lag in L than in K. The increase in lag during cycle B is attributable to an increase in nucleus luminosity, which was accompanied by partial sublimation of dust grains in a region cl oser to the center. The variable J and H emission is apparently determined not so much by dust radiation as by accretion-disk (AD) radiation.