On the relationship between the optical emission-line and X-ray luminosities in Seyfert 1 galaxies.
KRAEMER S.B., GEORGE I.M., CRENSHAW D.M. and GABEL J.R.
Abstract (from CDS):
We have explored the relationship between the [O III] λ5007 and the 2-10 keV luminosities for a sample of broad- and narrow-line Seyfert 1 galaxies (BLSy1s and NLSy1s, respectively). We find that both types of Seyfert galaxies 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, a fact 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 extreme-ultraviolet (EUV) continuum emission compared to BLSy1s. Also, other studies suggest that the gas in narrow-line regions (NLRs) of NLSy1s and BLSy1s spans 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 [O III]/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 NLRs of NLSy1s, resulting in lower average ionization; or (3) the presence of strong winds in the nuclei of NLSy1s that may drive off much of the gas in the NLR, resulting in lower cover fraction and weaker [O III] emission.