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Emission Line Diagnostics in a ``No UV Bump'' Quasar
PHL909
Smita Mathur, Belinda Wilkes, Martin Elvis
and Jonathan McDowell
Abstract
PHL909 is a quasar with unusually low ultraviolet luminosity (blue bump strength
C UV=IR = \Gamma0:20 compared to an average value of 0:4 (McDowell et al. 1989 ApJL 345, L11)).
It also has unusually flat hard X­ray spectrum. The UV spectrum of the object shows large
fluxes in the emission lines Lyff and CIV and the optical spectrum shows strong emission
lines of MgII and the Balmer series. The observed continuum is too weak to produce the
emission line fluxes unless the covering factor is very high (ё 50% c.f. ! 10% typical). The
intrinsic continuum of PHL909 ( i.e. as seen by the BELR ) may thus be similar to the
`standard' continuum, with a big blue bump, reddened along the line of sight.
Motivation
The quasar population as a whole covers a wide range of continuum properties such as
radio brightness, infrared breaks and most importantly the ionizing continuum: the UV blue
bump strength (McDowell et al. 1989) and X­ray brightness (Wilkes et al. 1994 ApJS, in press).
This range is sufficient to cause a marked difference in the physical conditions of the line
emitting gas; so using a ``standard quasar'' continuum obscures physics in the gas. The
relation between observed continuum and BELR physics can best be studied by examining
quasars with extreme characteristics in their continuum properties. Mathur (in IAU159 on
Multiwavelength Continuum Emission of AGN [Kluwer Academic] pp 271), Mathur et al. (1994
ApJ, in press ) have discussed the effect of X­ray quietness on the inferred parameters of
BELR clouds. In this paper we discuss emission lines properties of PHL909, a weak UV
bump quasar.
PHL909: Continuum Properties
The Spectral Energy Distribution of PHL909 shows no UV bump (Figure 1). The blue
bump strength CUV=IR = \Gamma0:20 compared to an average value of 0.4 ( McDowell et al. 1989).
The optical to X­ray power law slope ff OX = 1:3 compared to an average value of 1.5 ( Wilkes
et al. 1994). The hard X­ray spectrum is unusually flat: ff X = 0:4 (Average = 0.9 -- 1.2,
Williams et al. 1992 Ap.J. 389, 157). The X­ray spectrum shows excess in soft energies over
the hard X­ray slope (Masnou et al. 1992 A&A, 253, 35). It has an order of magnitude lower
number of ionizing photons in the observed continuum (3 \Theta 10 55 s \Gamma1 ) as compared to the
standard one (7 \Theta 10 56 s \Gamma1 ).

/proj/quasar/smita/qed/phl909/fig.ps.bb
Figure 1: The complete rest frame spectral energy distribution of PHL909 (data from Elvis
et al. 1994 Ap.JS, in press) corrected for Galactic reddening. The dashed line is the best
fit observed continuum. The solid line is the 'standard' continuum (as defined in CLOUDY,
Ferland 1991 OSU Astronomy department internal report)
PHL909: Emission Line Properties
The Lyff equivalent width of PHL909 is ё 100 љ A (Average = 65, Wilkes 1986 MN­
RAS 218, 331). The number of Lyff photons =1:3 \Theta 10 55 s \Gamma1 ) is thus comparable to the
number of ionizing photons in the observed continuum. The CIV/Lyff ratio is ё 1 (Average
= 0.63, Morton 1991 ApJS, 77, 119). The Hfi/Lyff ratio is ё 0:08 (Average = 0.22, Wills
et al. 1985 ApJ 288, 94). The Hfi equivalent width = 52 (Average = 47, Wilkes 1986).
Results
The observed continuum of PHL909 is unusual as described above. However, the emission
line spectrum is not unusual. What is the intrinsic continuum of PHL909? It could be:
(1) As Observed: The quasar may have unusual intrinsic continuum with no blue bump. This
requires that the ionization parameter U= 0.016 to get the observed line ratio CIV/Lyff.
The covering required to produce the observed line strengths is then very high (ё 50%).
This is highly unlikely. (2) Similar to the `Standard' but directed away from us: PHL909
may have the big blue bump in its continuum, but we do not see it. This may be a geometric
effect e.g. the accretion disk emitting in UV may be edge on (Cunningham, 1975 ApJ 202,
788). The difference in the UV luminosity between the standard and observed continua can be
attributed to the inclination effect in accretion disk models. But the spectrum is not observed
to harden as expected for the edge on accretion disk. It is thus unlikely that the observed
spectrum is just an inclination effect. (3) Similar to the `Standard' but reddened along the
line of sight: The column density of hydrogen required to produce the ``observed'' reddening
is ё 10 21 cm \Gamma2 . This absorber would show absorption lines in the UV and absorption in the
X­rays. The IUE data is not good enough to detect absorption lines. In X­rays, the soft
excess as observed in the Einstein spectrum (Masnou et al. 1992) may be due to an ionized
absorber. Einstein data is consistent with an ionized absorber along the line of sight with
the effective column density of neutral hydrogen = few times 10 21 cm \Gamma2 . This can be tested
with ASCA data.
If the intrinsic continuum is indeed standard, the covering factor deduced to produce the
observed line strengths = 0.06, as is usually observed in quasars (c.f. ! 10%) . Inferring
from the emission lines, the intrinsic continuum is highly likely to be standard,
with a big blue bump.