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Emission Line -- Continuum Correlations in Low­Redshift
Quasars
Belinda Wilkes, Smita Mathur, Jonathan McDowell (SAO) and Ari
Laor (Caltech)
We investigate the relation between the observed optical emission line strengths, widths and
ratios and the observed continuum properties for two samples of quasars: 43 Einstein observed
quasars for which we have obtained IR--soft­X­ray data to define the spectral energy distribution
(SED); 23 PG quasars: z!0.4; NH!1.9\Theta10 20 cm \Gamma2 of which a random subset of 10 has been
observed by ROSAT to date (Laor et al. 1994).
Introduction.
The strong, broad emission lines that characterise quasar spectra are generally believed to be
generated in gas photoionized by the central continuum source of the quasar, the broad emission
line region (BELR). Photoionization models, although reasonably successful in predicting the
average emission line properties of a quasar with a `standard', somewhat X­ray­loud continuum
shape, have not been tested with the wide range of quasar properties, either in the continuum
(Elvis et al. 1994) or the emission lines. The emitted line strengths are sensitive to SED shape,
particularly in the ultraviolet (UV)/soft X­ray and IR bands (Krolik and Kallman 1988, Ferland
and Persson 1989, Mathur et al. 1994). Thus, if photoionization models are generally applica­
ble, we would expect systematic relations between the observed lines and continuum in different
objects. Along with our collaborators, we have now collected sufficient SED and emission line
data to study continuum/line relations for individual objects (Mathur et al. these proceedings)
and for samples, as we report here.
PG Sample Results
The following significant correlations were found in the initial subset of 10 quasars from this
sample (Laor et al. 1994). References indicate earlier reports of a similar correlation/trend in
each case (L x = X­ray luminosity at 2 keV)
L(FeII)/L(Hfi) ¸ ff x Wilkes, Elvis and McHardy (1987)
FWHM(Hfi) ¸ ff x Puchnarewicz et al. (1992)
L([OIII]) ¸ ff x
L(Hfi) ¸ L x Kriss et al. (1980), Blumenthal et al. (1982)
L([OIII]), L(HeII), L(FeII) ¸ L x Boroson and Green (1992), HeII¸L opt ,ff ox
The small sample size prevents multivariate analysis at this stage so it is not clear whether
the X­ray or some other parameter (L opt (Optical luminosity at 2500 š A), ff ox (effective optical­to­
X­ray slope, 2500 š A-- 2 keV)) is the fundamental one with which the lines correlate. Similarly,
since FWHM(Hfi) and L(FeII) also correlate (Zheng and O'Brien 1990), it is not clear which is
the fundamental correlation between FWHM(Hfi), L(FeII) and ff x .
Einstein Sample
The continuum parameters measured for the Einstein sample are as follows: L opt ; L x ; ff ox ;
ff x (energy index of soft X­ray spectrum); C UV=IR (blue bump strength: log[L UV /L IR ], McDowell
et al. 1989); R L ?! 1 (radio­loud/quiet).
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The following correlations/lack of correlations were found in the data. Multivariate analysis
was performed to determine the dominant correlation in each case:
L(MgII) ¸ L x , L opt , ff ox L x is fundamental
L(Hfi), L(Hfl), L(Hffi) ¸ L x , L opt , ff ox L x is fundamental
Line L 6¸ ff x , C UV=IR No relation between lines and blue bump
W – 6¸ L x , L opt , ff ox ff x , C UV=IR No Optical Baldwin effect (Zamorani et al. 1992)
W – (FeII) 6¸ ff x (R L ! 1) Contrary to PG sample and Shastri et al. (1993)
Discussion
While these results generally agree with earlier correlations found between line and continuum
properties, we also conclude that the X­ray luminosity is more strongly correlated with the low
ionization line luminosities than the optical luminosity or ff ox . This provides support for models
in which the low­ionization lines are generated at high optical depth in the emitting gas where the
ionization structure is determined predominantly by X­ray heating (Netzer 1990). The lack of a
correlation with the blue bump is understandable in the same scenario. This predicts that the
high­ionization UV lines should be more sensitive to the blue bump (and soft X­ray emission).
Alternatively it is possible that the BELR `sees' a different continuum than that which reaches
us (as for PHL909, Mathur et al. these proceedings). Our study will now be extended to include
the UV lines in order to discriminate between these two scenarios.
The existence of a relation between FeII–4570 and the soft X­ray slope has been disputed for
some time (Wilkes, Elvis and McHardy 1987, Zheng and O'Brien 1990, Boroson 1989, Shastri
et al. 1993). The results are conflicting even in the two samples presented here: positive for the
PG sub­sample and negative for the Einstein sample. It is likely that the correlation applies to
some sub­class of AGN that has not yet been identified by other properties.
References
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