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Optical Continuum Variability of the Radio Galaxy 3C-390.3

Abstract

Continuum light curves in four wavelength bands (Johnson/Cousins B, V, R, and I) are presented for the broad-line radio galaxy 3C-390.3. The observations span the period 2001 to 2021, and were obtained with telescopes operated by the Southeastern Association for Research in Astronomy (SARA) in Arizona and on the island of La Palma. These data, combined with spectroscopic observations obtained by other observers, allow determination of the mass of the central supermassive black hole (currently estimated at 4.34 ± 0.43 × 108 MSun) by the method of reverberation mapping. During the period 2001 to 2021 the galaxy 3C-390.3 exhibited three cycles of brightening and dimming, with diminishing amplitude of variation. The galaxy brightened rapidly during the second half of 2021. There is a correlation between luminosity and color: the brighter the galaxy, the bluer it appears. This behavior is commonly observed in active galaxies. A correlation between the mass of an active galaxy’s central black hole and the characteristic time scale with which its luminosity diminishes after a brightening event was reported in August 2021. This would provide a highly valuable tool, as it would in principle allow determination of a supermassive black hole’s mass solely from photometric observations. However, the light curves for 3C-390.3 presented here suggest that this “damping time” depends on the optical band observed, increasing with increasing wavelength.

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