A new activity cycle of NGC4151: Evidence for the existence of an accretion disk

Astronomy and Astrophysics – Astronomy

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Scientific paper

New UBV observations of the nucleus of the Seyfert galaxy NGC 4151 are presented. The light curve in forty years is analyzed. It is noted that after the extended photometric minimum of 1984-1989, when the contribution of the variable source to the total flux was nearly zero, a new activity cycle began. Between 1989 and 1996, a significant brightening was observed: the U brightness through a 27.5" aperture rose by 2.5 mag. Based on multiaperture photometry, we allow for the stellar emission from the underlying galaxy and separate the variable source in the nucleus. All published UBV observations are also used for the analysis. A colorimetric analysis of the variable source shows that the variable source at the new active phase (1990-1998, cycle B) differed from that operated in 1968-1983 (cycle A) by its characteristics. First and foremost, the luminosity of the variable source at the maximum of cycle B was twice its luminosity at the maximum of cycle A, while the color indices were the same. This strongly suggests that the variable source increased in size. The color characteristics of the variable source in the two cycles are generally the same, but the dispersion of B-V in cycle A is a factor of 3 or 4 larger. In general, the color indices of the variable sources in cycles A and B change with luminosity in a similar manner. At minimum and maximum light, they corresponded to a blackbody radiation at temperatures of ~ 8000 K and ~ 30 000 K, respectively; a considerable ultraviolet excess was observed in the latter case. The disappearance of the old variable source and the appearance of a new variable source with slightly different characteristics in a mere ~10 years can be explained by changes in the accretion disk around the supermassive compact object (a black hole?) in the galactic nucleus.

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