The mechanisms that make some quasars radio-loud and others
radio-quiet are not well understood but the major differences may be
attributable to the spin of the black hole
(Wilson & Colbert 1995).
There are a number of well established distinctions between the two
classes. (i) Radio-loud quasars are associated with elliptical host
galaxies and radio-quiet quasars tend to reside in spiral galaxies.
The mean absolute magnitude of the underlying galaxies of radio-loud
quasars is similar to that of radio galaxies, however the host
galaxies of radio-quiet quasars are 0.6-1.0 magnitudes less luminous
(Smith et al. 1986;
Véron-Cetty & Woltjer 1990). Recent deep near
infrared (K-band) imaging of host galaxies of quasars revealed that
more than half of RQQSOs appear to lie in galaxies that are dominated
by an exponential disk
(Taylor et al. 1996). Those RQQSOs that have
elliptical host galaxies show signs of interaction and are in general
more luminous than those that reside in disk galaxies. There is also
some evidence to suggest that a large majority of low-luminosity
radio-quiet AGN lie in disk galaxies but a significant fraction of
RQQSOs more luminous than 
have elliptical
host galaxies. (ii) Unlike RQQSOs, radio-loud quasars produce large
scale jets and lobes and can be defined by their radio luminosity -
those with
are
classified as RQQSOs, and those with
as
radio loud quasars
(Miller et al. 1990). (iii) Gamma-ray results
recently obtained by the Compton Gamma Ray Observatory (CGRO) have
produced evidence for two classes of AGN
(Dermer & Gehrels 1995).
These classes are defined by their redshift, luminosity distributions
and high energy spectral properties. The first class of objects have
redshifts 
and 50-150 keV luminosities in the range
. Associated with this group are
Seyfert galaxies, RQQSOs and radio-galaxies viewed at large angles
with respect to the radio jet axis. The gamma-ray spectra for these
sources soften between 
and several MeV. The
second class of source consists of blazars with redshifts as large as
2.3 which have detectable fluxes of gamma-rays above 100 MeV
with emission extending into the GeV band. This class of source
probably consists of AGN that are observed close to the axis of a
radio jet. So far no RQQSO has been detected at gamma ray energies
above 10 MeV.
Unlike radio-loud quasars, intensive optical monitoring of RQQSOs only
started in the early 1990s. Gopal-Krishna et al. (1995) monitored a
sample of six optically bright and luminous RQQSOs
(
and MV<-23) and found strong
hints for variability in three RQQSOs; PG 0946+301
displayed an R-band increase of 
magnitudes in a time of
; PG 1049-006 varied in V-band by
magnitudes in 
and PG 1206+459
varied by 0.04 magnitudes in 
in the R-band.
Sagar et al. (1996) reported that the flux density from
PG 1444+407
dropped by 0.04 magnitudes in in the R-band.
Sagar et al. (1996) also reported long-term variability (over
) for four RQQSOs in their sample, with the largest
variability of about 0.15 magnitudes in 11 months, recorded for the
source PG 1049-005.
Jang & Miller (1995) reported
intranight variability in the RQQSO II Zw 175 which varied by
magnitudes over a period of 4 days. The reported detection
of variability in RQQSOs is not in conflict with the results presented
here and in Tables 1 (click here) and 2 (click here),
because the small levels of variability would not have been detected
above the 
level in many of the sources monitored in this
survey. One source, PG 2112+059, displayed long-term
variability decreasing by 0.18 magnitudes in the V-band over a period
of almost four years.
The long term variability of large samples of optically-selected quasars have been studied over decades (Hook et al. 1994; Cristiani et al. 1996). In these samples, a strong negative correlation between variability and quasar luminosity was found with the more luminous quasars displaying less variability. This result is interesting considering that RQQSOs in this sample all have high luminosities (-27<MV<-30), and no rapid optical or night to night variability was detected in any of the sources.
The radio spectra of RQQSOs probably have contributions from three
components: (i) optically thin synchrotron from star forming regions
in the disk of the host galaxy and in a circumnuclear starburst, (ii)
optically thin synchrotron from an extended or possibly jet-like
component powered by an active nucleus and (iii) partially opaque
synchrotron from a compact VLBI-scale core. In an extensive survey of
RQQSOs, Kellermann et al. (1994) obtained VLA maps which show extended
and double lobe radio structures in some sources that are similar to
those observed in radio-loud quasars. The RQQSOs mostly have a radio
luminosity well in excess of the 
found for
most normal spiral and elliptical galaxies and hence the radio
emission is not simply that from the underlying galaxy. It is quite
possible that for sensitivity reasons quasars with additional
low-surface brightness features may have been missed in the VLA
mapping of the radio-quiet sources. Nevertheless, a number of the
radio-quiet quasars in the
Kellermann et al. (1994) survey (e.g.
0953+41; 1116+21; 1634+70) are well
resolved and show extended structure ranging between 49 and
. Recent VLA results
(Barvainis et al. 1996)
revealed heterogeneous spectral shapes in the radio spectra of a sample
of RQQSOs that could be classified into general categories similar to
radio loud quasars. Furthermore variability was discovered for seven
sources most of which had flat or inverted radio spectra. In one
source VLBI revealed that essentially all the flux emanated from one
compact sub-parsec core. The radio results on these types of sources
appear to be inconsistent with starburst models and imply that the
cores of many RQQSOs may be scaled down versions of those found in
radio loud quasars (Stein 1996).
Recent new results have highlighted the unusual properties of two
RQQSOs, both of which lie near the top of the upper band of radio
emission for RQQSOs (
). The first source IRAS
13349+2438 was initially detected through its strong infrared
emission (Beichman et al. 1986) and has been classified as a
radio-quiet, infrared bright quasar with a value of R=1.9. It has a
redshift of z=0.107 and a high polarisation which rises from
1.4% at 2.2 
m (K-band) to 8% at
0.36 m (U-band).
Wills et al. (1992) found no variability
of the polarisation or flux density on timescales from days to months
and discussed a bipolar geometry to account for its polarisation and
other properties. VLA observations revealed an unresolved source with
an unusual radio spectrum with a maximum flux density of 7 mJy
at a frequency of 5 GHz. The origin of the peaked radio
emission is not understood but absorption of the radio emission may
occur in the dusty dense interstellar medium and also the
contributions of different spectra from several source components may
be involved.
IRAS 13349+2438 has high polarisation, strong Fe II emission and is radioquiet but no broad absorption lines (BAL) have been observed. IRAS 13349+2438 has been observed on several occasions by ROSAT where the source was found to vary by a factor of 4.1 in about one year and about 25% in one week (Brandt et al. 1996) but showed no evidence for the large intrinsic absorption of soft X-rays by cold neutral matter. The soft X-ray variability excluded electron scattering for most of the soft X-rays and suggest absorption by a warm ionized gas with internal dust.
Recent ASCA observations of IRAS 13349+2438 discovered for
the first time rapid X-ray variability or blazar like behaviour in a
RQQSO. The source displayed intensity variations on two separate
occasions by factors of two on timescales of only a few hours without
any significant spectral changes
(Brinkmann et al. 1996). There is
also some evidence in the X-ray data for even more rapid variability.
The 0.6 to 8 keV spectrum was fitted with a power law with
which is steeper than the average value of 
found for RQQSOs in the ASCA band.
Brinkmann et al. (1996) point out
that the line of sight to the quasar may graze the edge of the torus
and suggest that small changes in viewing conditions could produce
marked changes in intensity and spectral shape. However it is
difficult to understand how changes in viewing conditions could
produce such rapid variability with no significant spectral changes.
It is plausible that IRAS 13349+2438 is an example of a radio
loud quasar that is viewed through a dusty ionizing outflow, possibly
associated with a merger, that severely attenuates the radio emission
so that the source is classified as a RQQSO. In this context it should
be noted that the quasar III Zw 2 could be classified as a
RQQSO at 5 GHz but the radio spectrum rises steeply toward
higher frequencies and this source is radio-loud when the 90 GHz
flux density is adopted
(Schnopper et al. 1978). Further observations
of IRAS 13349+2438 across the full spectrum including VLBI
searches for a compact self-absorbed component or indeed multiple
source components may help elucidate the nature of this unusual hybrid
type source.
The second unusual RQQSO is PG 1416-129 at z=0.129. It
has been classified
(Turnshek & Grillmair 1986;
Ulrich 1988) as a
broad absorption line quasar
(BALQSO). The value of R is
1.1 and similar to IRAS 13349+2438 it lies near the top of
the upper band for RQQSOs which is also heavily populated with BALQSOs
(Francis et al. 1995).
This source has a soft X-ray excess
(de Kool & Meurs 1994) and the hardest spectral index of any source in the energy
range 2-20 keV when observed with GINGA
(Williams et al. 1992).
Staubert & Maisack (1996) detected this bright RQQSO at
energies above 50 keV with the OSSE telescope on CGRO and the
flux was found to be variable on a timescale of days during the 14 day
observation.
The BAL classification of this source has been questioned by
Green & Mathur (1996) who proposed that large values of the optical to X-ray
slope (
) be the defining characteristic of
BALQSOs. They report a low value of (
) for
PG 1416-129 and suggest further observations with HST to
check the BAL classification. VLA observations of
PG 1416-129
reveal unusual results for a RQQSO: (i) the radio source consists of
an unresolved core coincident with an extended component that is
assumed not to be an unrelated background source
(Kellermann et al. 1994), (ii) the source varied by a factor of 4.5 over a period of ten
years and could have been more variable given the very limited
monitoring at radio frequencies and (iii) the radio spectrum is
consistent with a flat or inverted spectrum
(Barvainis et al. 1996).
No VLBI observations have been reported for PG 1416-129 but
one of the other variable RQQSOs PG 1216+069 was detected
with essentially all of its flux in the VLBI core and the high
brightness temperature limit confirmed the self-absorbed synchrotron
hypothesis for the flat spectrum component
(Barvainis et al. 1996).
This component may well be a scaled-down version of the radio sources
observed in radio loud quasars and future VLBI observations of
PG 1416-129 may also reveal a similar component.