The narrow [O iii ] λλ4959, 5007 emission-line fluxes in the spectrum of the well-studied Seyfert 1 galaxy NGC 5548 are shown to vary with time. From this we show that the narrow-line-emitting region has a radius of only 1–3 pc and is denser ( n e ∼ 10 5 cm −3 ) than previously supposed. The [O iii ] line width is consistent with virial motions at this radius given previous determinations of the black hole mass. Since the [O iii ] emission-line flux is usually assumed to be constant and is therefore used to calibrate spectroscopic monitoring data, the variability has ramifications for the long-term secular variations of continuum and emission-line fluxes, though it has no effect on shorter-term reverberation studies. We present corrected optical continuum and broad Hβ emission-line light curves for the period 1988–2008.
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A 2013 study studied this question.