The Fading of the Narrow-Line Region in 3C 390.3

Abstract

IUE spectra of 3C 390.3 obtained at 17 different epochs from 1978 to 1986 show that the flux in the narrow core of the Ly-alpha 1216 A and C IV 1550 A emission lines steadily declined by a factor of 1.7 and 3.2, respectively. Lloyd's optical monitoring reported in 1984 shows that the continuum intensity decreased by 2 mag over the years 1968-1980. Together, the UV and optical light curves form the strongest evidence to date that the NLR is photoionized and reverberates the continuum flux. Moreover, an upper limit of 10 lt-yr can be set for the size of the narrow line region (NLR) in this radio galaxy. The gas density is also unusual, and probably reaches 10 to the 7th/cu cm, at least in the inner part of the NLR. The broad-line gas, on the other hand, lies at only 10 to the 17th cm from the source of continuum and responds almost immediately to a change of its intensity.