Infrared variability of the nucleus of the Seyfert galaxy NGC 1068 in 1998–2006

摘要

The results of analysis of JHKLM photometry of the Seyfert galaxy NGC 1068 are presented. These observations confirmed its infrared (IR) variability. The magnitudes of the amplitudes of the brightness variations in the J (1.25 μm) and K (2.2 μm) bands are within 0.15 and 0.3, respectively. The nucleus of NGC 1068 is a variable source and can be at different phases of activity. The variable source in NGC 1068 is a complex-structured object. At least two sources radiate in the wavelength range 1.25–5 μm: a hot source whose radiation shows up in the range 1.25–1.65 μm and a cold source radiating at long wavelengths (2.2–5 μm). The total mean luminosity of the two sources did not change between the beginning and the end of our observations. The IR brightness and colour variations observed in 1998–2006 are attributable to the dispersal of the dust envelope that formed around the galactic nucleus some 30 years ago and reached its maximum density in 1994–1995. The optical depth of the dust envelope averaged over the spectrum of the hot source is τ ≈ 1.5. In 2004, the state of the dust envelope almost returned to its 1974 level, i.e. the dust envelope formation and dispersal cycle was about 11 000 days (approximately 30 years).