The Size of the Narrow-Line Emitting Region in the Seyfert 1 Galaxy NGC
  5548 from Emission-Line Variability
  The narrow [O III] 4959, 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 ~ 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 Hbeta
emission-line light curves for the period 1988 to 2008.
