The Variability of Optical Narrow Lines in 3C 390.3

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Abstract

Based on the optical spectroscopic data, we study the intensities of the narrow lines in broad-line radio galaxy 3C 390.3 between 1974 and 1990. The flux of [O III] λ4959 has declined by ~40% between 1974 and 1984. It has remained at a low level between 1984 and 1987, and then increased to a higher level since 1988, in step with the long-term variation of the underlying optical continuum and with the reported fading of narrow core of the Lyα and C IV emission line between 1978 and 1986. Differential variations are found among narrow lines, following a pattern in which low-ionization lines, such as [O I] λ6300 and [N II] λ6583, become relatively stronger. A significant change in the [O III] λ4363/4959 ratios took place around 1980, and the Hβ/Hα ratio has varied between 0.25 and 0.40, suggesting that we have observed emission from regions with different densities and extent of reddening. In 1984 when the continuum reached a minimum level, the fluxes of narrow lines declined within four months by ~40%. The short time scale of variations implies that the narrow-line region in this object must be compact (~1 lt yr) and dense (n ~> 10^6^ cm^-3^). The most probable scenario is the alignment of an elongated narrow-line region close to the line of sight, which would respond to the continuum variation without a significant lag.

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