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On the Relationship Between the Optical Emission-Line and X-ray Luminosities in Seyfert 1 Galaxies
We have explored the relationship between the [O III] 5007 and the
2--10 keV luminosities for a sample of Broad- and Narrow-Line Seyfert 1
galaxies (BLSy1 and NLSy1, respectively). We find that both types of Seyferts
span the same range in luminosity and possess similar [O III]/X-ray ratios. The
NLSy1s are more luminous than BLSy1s, when normalized to their central black
hole masses, which is attributed to higher mass accretion rates. However, we
find no evidence for elevated [O III]/X-ray ratios in NLSy1s, which would have
been expected if they had excess EUV continuum emission compared to BLSy1s.
Also, other studies suggest that the gas in narrow-line regions (NLR) of NLSy1s
and NLSy1s span a similar range in ionization, contrary to what is expected if
those of the former are exposed to a stronger flux of EUV radiation. The
simplest interpretation is that, like BLSy1s, a large EUV bump is not present
in NLSy1s. However, we show that the [OIII]/X-ray ratio can be lowered as a
result of absorption of the ionizing continuum by gas close to the central
source, although there is no evidence that intrinsic line-of-sight absorption
is more common among NLSy1s, as would be expected if there were a larger amount
of circumnuclear gas. Other possible explanations include: 1) anisotropic
emission of the ionizing radiation, 2) higher gas densities in the NLR of
NLSy1s, resulting in lower average ionization, or 3) the presence of strong
winds in the the nuclei of NLSy1s which may drive off much of the gas in the
narrow-line region, resulting in lower cover fraction and weaker [O III]
emission.Comment: 18 pages, including 3 figures, 2 tables. Accepted for publication in
The Astrophysical Journa