% 2000-07-12: Started to write the draft in the AJ format / AASTEX v5.05
% 2000-10-31: Submitted to AJ.
% 2000-11-09: Revision started.

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\slugcomment{Revised version submitted to the Astronomical Journal: 
November 18, 2000}

\shorttitle{Gigahertz Peaked-Spectrum Radio Sources and Candidates}
\shortauthors{Tornikoski et al.}

\begin{document}

\newcommand\thour{\mbox{$^{\mathrm h}$}}
\newcommand\tmin{\mbox{$^{\mathrm m}$}}
\newcommand\tsec{\mbox{$^{\mathrm s}$}}

\title{Radio Spectra and Variability of Gigahertz Peaked-Spectrum 
       Radio Sources and Candidates}


\author{M. Tornikoski\altaffilmark{1},
        I. Jussila and 
        P. Johansson\altaffilmark{2}}
\affil{Mets\"ahovi Radio Observatory, Mets\"ahovintie 114, FIN--02540 
         Kylm\"al\"a, Finland}
\author{M. Lainela and 
        E. Valtaoja\altaffilmark{3}}
  \affil{Tuorla Observatory, FIN--21500 Piikki\"o, Finland}

\altaffiltext{1}{
  Email: Merja.Tornikoski@Hut.Fi}
\altaffiltext{2}{
  Present address: Observatory, University of Helsinki, P. O. Box 14,
  FIN-00014 University of Helsinki, Finland}
\altaffiltext{3}{
   And: Dept. of Physical Sciences, University of Turku,
   FIN--20100 Turku, Finland}

%-------------- ABSTRACT:

\begin{abstract}
We have made long-term high radio-frequency observations of Southern
and equatorial active galactic nuclei (AGNs). After complementing
these data with data from the literature we have constructed the radio
spectra of these sources and searched for sources with inverted
spectra in the gigahertz range. We have identified 12 new sources with
spectral shapes resembling those of the gigahertz peaked-spectrum (GPS)
sources, and 8 other sources with inverted spectral parts in the
gigahertz region.  Several of these new GPS source candidates
have high ($\geq 10$ GHz) peak frequencies in the observer's frame,
and they all exhibit strong long-term variability. 
We have also studied the variability behavior of known GPS
sources, mainly quasar-type sources.  
All of the 14 sources included in our sample show moderate
to extremely high radio variability, and at least 12 of them are variable
also in the millimeter domain. Long-term monitoring of these sources
shows that some of them have spectral shapes resembling those of
classical variable flat-spectrum sources, indicating that 
some of these sources have been misidentified as GPS sources when
only sparsely sampled data have been available. On the other hand,
4 of the variable sources show persistent GPS-type spectra at all 
stages of activity, suggesting that at least some of the GPS
sources can be variable.
\end{abstract}

\keywords{quasars: general -- radio continuum}


%-------------- TEXT:

\section{Introduction}

Gigahertz Peaked-Spectrum (GPS) radio sources are extragalactic radio
sources characterized by convex spectra peaking in the gigahertz range
and steep at high radio-freqeuencies.  Even though there are several
other properties often coupled with the definition of a GPS source,
the spectral shape is the single most important characteristic of a GPS
source.  A comprehensive review of the properties of GPS sources has
been given by \citet{odea98}.

Most known GPS sources peak at around 1 GHz \citep{devries95}.  Only
few known sources peak at extremely high frequencies, above 10 GHz.
\objectname[]{RXJ~0952.7+5151} and \objectname[]{RXJ~1459.9+3337}
peak at 20 and 30 GHz, respectively \citep{edge96, edge98}, and
\objectname[]{B1936$-$155} and \objectname[]{B2008$-$159} both
peak at $\nu_T \approx$10--20 GHz in the observer's frame
\citep{tornikoski2000}.  \citet{snellen98a}
report the source \objectname[]{B1945+6024} peaking at a frequency higher
than 15 GHz, because the highest observing frequency that their group
used was 15 GHz, and the source spectrum was still inverted at that
frequency.  

In addition, \citet{grainge} report the detection of 50 GPS sources
with extreme peak-frequencies in a survey area of 2000 square degrees.
The peak-frequencies of these sources were above 10 GHz, and several
of them had rising spectra at the highest observing frequency of 43
GHz. The sources included in this survey were relatively faint (a flux
limit of 50 mJy at 5 GHz was used), i.e. sources that often are
excluded from high-frequency studies. A fuller report of this study,
or a source list, are not available yet, but the result suggests that
the number of extreme-peaking GPS sources is much greater than earlier
assumed. Indeed, so far the selection has mainly been made using 
low-frequency data only, so the population of sources that peak above
ca. 10 GHz could be strongly underrepresented in the 
centimeter-wavelength-selected samples.

In addition to the aforementioned fact that sources relatively faint at
low radio-frequencies easily are excluded from high-frequency studies,
also the very low number of data points may play a role in the
selection process.  Selecting GPS-candidates based on one-epoch
observations is problematic if the candidate is variable,
because it is impossible to determine whether the source spectrum
has a true GPS shape or whether the source was only observed in a very
active state.  In many radio loud quasars the spectrum at high 
radio-frequencies considerably flattens, or even gets inverted, during an
outburst.  Also when the GPS candidate selection for higher frequency
studies is made by using catalog data with data entries possibly
from more than a decade earlier than the high-frequency observation,
the nonsimultaneity of observations makes it impossible to determine
the true shape of the radio spectrum.

In \citet{tornikoski2000} we suggested that many more GPS sources with
extreme peak-frequencies exist, but they are easily ignored because
their low-frequency data points may have been taken during a flaring
state of their variability cycle, indicating flatter spectra than the
typical inverted spectra of the canonical GPS sources, and especially
because they are not being systematically searched for at the high
radio-frequencies. 

Our group has been studying AGNs at high radio-frequencies since 1987
with the Swedish--ESO Submillimeter Telescope (SEST) at the European
Southern Observatory (ESO) site of La Silla, Chile, latitude
$29\arcdeg$ South. During our various observing projects we have
observed both well-known strong and variable sources as well as some
samples of rarely observed sources (e.g., \citet{tornikoski93,
tornikoski94, tornikoski99a, tornikoski2000}; details about the SEST
AGN monitoring observations can be found in \citet{tornikoski96}).  
We have not made any systematic search for new GPS sources, but from our
relatively well-sampled data sets complemented by data from the literature
we were earlier able to identify two new GPS sources peaking at high
frequencies \citep{tornikoski2000}, and that encouraged us to
thoroughly examine our complete data base in order to identify more
high-peaked GPS sources and candidates among the sources observed
with the SEST.

The sources currently included in the GPS source lists are very
heterogeneous. Nevertheless, there are many observational
characteristics often coupled with the definition of a GPS
\citep[and references therein]{odea98}:
1. They are very compact, with linear sizes of $<$1 kpc.
2. In the radio, these sources together with the compact steep
spectrum (CSS) sources are the least variable class of compact
extragalactic radio sources.
3. Superluminal motion appears to be rare.
4. Optical polarizations are low ($\leq1\%$).
5. GPS sources identified with quasars often have large redshifts.

Even though the aforementioned properties seem to be common to most
GPS sources, there are counter examples for all of them \citep{
edge96, edge98, snellen95, odea91, odea98}.  Thus the shape of the
spectrum is the most important feature presently known that
distinguishes the GPS sources from other compact extragalactic radio
sources.

The spectra of GPS sources tend to be steep but fairly straight at
frequencies above the peak frequency.  \citet{devries97} have
constructed a generic GPS source spectrum based on their sample of
sources peaking between 0.5 and 10 GHz. For this spectrum the
optically thick spectral index is $\alpha=+0.51\pm0.03$ (for
$S\propto\nu^\alpha$), and after the turnover $\alpha=-0.36\pm0.05$
for the flat part $\nu_T<\nu\leq 2\nu_T$ and $\alpha=-0.73\pm0.06$ for
$\nu> 2\nu_T$.

In Section~\ref{sec:candidates} we discuss the radio spectra of a set of
sources and identify new GPS candidates, in Section~\ref{sec:variability} we
discuss the radio variability of GPS sources and in
Section~\ref{sec:discussion} we discuss the properties of the GPS sources in
the light of our observations.

%-------

\section{Radio spectra of new GPS source candidates}
\label{sec:candidates}

In \citet{tornikoski2000} we studied the spectra and variability
of a complete sample of flat
spectrum ($\alpha_{2.7GHz-5.0GHz}>-0.5$) bright ($S>1$Jy at least once
at frequencies $\nu \ge 10$GHz) sources in the K\"uhr 1 Jy catalog
\citep{kuhr81} and within the declination range of
$0\arcdeg>\delta>-25\arcdeg$. In that paper we also identified two new
high-peaking quasar-type 
GPS sources, \objectname[]{B1936$-$155} and
\objectname[]{B2008$-$159}.

For this paper, we wanted to study additional objects not included in 
\citet{tornikoski2000}.
Out of our SEST data base, we have examined $\sim$100 sources
with at least one data point at 90 GHz and/or 230 GHz with $S/N\ge4$.
Almost all of these sources are quasars or BL Lacertae objects.
Many of these sources are standard targets of radio or multiwavelength
monitoring and are known to exhibit strong variability and have 
relatively flat radio spectra. Thus it was not surprising to 
find a large number of sources having spectra that were very flat
or even inverted in the cm--mm region, especially during outbursts. 
In order to discriminate against true GPS-type spectra and spectra
that are inverted only during an outburst, we gathered as much
cm- to mm-data as possible. We complemented our SEST data sets 
with the following: the Parkes Catalog \citep{pks}; hereafter PKS;
the K\"uhr 1Jy catalog \citep{kuhr81}; hereafter K1; and the 
following other published data of one or several sources: 
\citet{beasley, bramwell93, edelson87, ghosh94, giacani88, gregorini84, 
kovalev99, kuhr81, quiniento88, quiniento90, quiniento93, reuter, salonen, 
steppe92, steppe93, 
% Huom! steppe88 ei mukana iram-koosteissa!
terasranta92, terasranta98, tingay97, tornikoski93, tornikoski96, veron96, 
pks, umrao}.
The frequency range of these data extends from
$\nu<$1 GHz to $\nu\approx$ 230 GHz.
The references and frequencies are listed in
Table~\ref{tab:datainfo}.
For many of the sources we had several epochs of data points
across the entire frequency range. The longest and best-sampled
time series come from our Mets\"ahovi and SEST observations
that span for over 15 and 12 years, respectively.

In the following, we examine in more detail the sources that were
found to have a clearly convex, GPS-type, spectrum
(Subsection~\ref{subsec:new_gps}), and some other sources
that seem to have an inverted spectrum, but which is
flatter than the spectra of the typical GPS sources, or for which we have too
little data to draw any definite conclusions about their true spectral
shape (Subsection~\ref{subsec:cand}).  In
Table~\ref{tab:cand_gps_info} and Table~\ref{tab:cand_inv_info} we
list these sources, respectively, together with their classification
and redshift information.  
We use the following classification for the
sources: BLO stands for BL Lacertae objects, HPQ for highly polarized
quasars with a measured optical polarization $p>$3\%, LPQ for low
polarized quasars with a measured optical polarization $p<$3\%,
QSO for quasars with no polarization information available, and 
GAL for galaxies.  For the
sources in Table~\ref{tab:cand_gps_info} we also list the estimated
peak frequencies of their spectra, estimated from the quiescent
spectra (minimum fluxes) and assuming the spectral shape of the
``generic'' GPS spectrum by \citet{devries97}, when applicable.

When studying the radio spectra of quasars
and trying to
determine whether the source has a GPS source spectrum or not, for
example by comparing the shape to the generic GPS source spectrum
presented by \citet{devries97}, we must bear in mind that many of
these sources exhibit very strong radio variability which often
flattens the shape considerably, or even turns an otherwise falling
spectrum into an inverted one during an outburst.  The variability
will be discussed in Section~\ref{sec:variability}, and in this
section we will mainly concentrate on studying the minima of the
measured flux densities at each frequency, i.e. the assumed quiescent
spectrum. For the sources with very few data points (or, in some
cases, only one-epoch data at each observing frequency) the
variability behaviour is unknown and thus we can not make any
conclusive statements about the true shape of their radio spectrum.

\subsection{Sources with GPS-type spectra}
\label{subsec:new_gps}

In Fig.~\ref{fig:newgps1} and Fig.~\ref{fig:newgps2}
we have plotted the radio spectra of all of the sources that exhibit
GPS source -type spectra. The data for these plots are as listed in 
Table~\ref{tab:datainfo}.
We have left out any measurements with $S/N<4$. The catalog data
points with no error bars contained no information about the error of the
measurement.

% NOTE: \objectname[]{} does not work inside the \paragraph-command!

\paragraph{B0048$-$097.} 
% BLO
% z unknown, lower limit 0.2
This is a relatively well-studied BL Lac object,
with strong variability across the whole frequency range. The old catalog
data from PKS \& K1 seem to represent a relatively high state of the
activity cycle. Even the shape of the quiescent spectrum, however, remains 
slightly inverted up to 10 GHz. During the outbursts the spectrum is
highly inverted up to ca. 10 GHz, from where it remains relatively
flat up to 90 GHz.

\paragraph{B0332$-$403.} 
% HPQ
($z=$~1.445 often quoted, but \citet{shen98} point 
out that this redshift, though widely used in the literature, may be 
incorrect).
This source is an HPQ ($p=$~14.8\%, \citet{impey90}). 
This source has not been listed in the published lists of GPS sources
or candidates, but based on the comment by \citet{shimmins71}
and the generally accepted definition of a GPS, \citet{shen98} mention
that this source could be classified as a gigahertz peaked-spectrum source.
Our data confirm the GPS-nature of its spectrum, showing a
spectrum that is inverted up to 5 GHz.

\paragraph{B0454$-$234.} 
% HPQ
($z=$~1.003)
This is an HPQ, with an optical polarization up to $p=$27\% \citep{wills92}.
The source exhibits strong variability from 2 to 230 GHz.
Based on only one-epoch but simultaneous observations at 1 and 3.9 GHz
\citep{kovalev99}, the radio spectrum of 
this source seems to be inverted to ca. 3--4 GHz. During outbursts the
spectrum is inverted all the way up to 20 GHz, but in the quiescent
state the spectrum steepens after 8 GHz.

\paragraph{B0537$-$441.} 
% BLO
($z=$~0.894) Usually classified as a BL Lac
object, but also referred to as a transition object between classical
BL Lac objects and quasars \citep{maraschi85}.
The source is rapidly variable across the electromagnetic
spectrum, and it is a strong variable EGRET $\gamma$-ray source
\citep{thompson93, hartman99}.
The source has been proposed as a candidate for microlensing
\citep{stickel88, romero95}, but the microlensing theories have also
been disputed by other authors \citep{falomo92, lewis97}.
In our plot the spectrum is inverted up to ca. 5 GHz and during outbursts
up to at least 10 GHz.

\paragraph{B1057$-$797.} 
% QSO
This is a relatively little observed very southern source with no data
available between 8 and 90 GHz. With the currently available data the
spectrum appears inverted up to at least 8 GHz. There is no polarization
information available for this source and no information about its
redshift.
% I&T 1990, mainittu, ei mitattu
% Beasley: nimell� J1058-8003! Data lis�tty omaan excel-taulukkoon.
% PKS: z=3.04, I&T:n mukaan ei tunneta?

\paragraph{B1334$-$127.} 
% HPQ
($z=$~0.539) An HPQ with $p=$16.1\% \citep{kuhr90}.
This source has been well sampled at the SEST observing frequencies
and shows very strong and rapid variability at the high radio
frequencies. With the currently available data set the quiescent
spectrum is slightly inverted up to 20 GHz. At 90 and 230 GHz the
source has been monitored very densely between 1987 and 1999, and the
data set probably includes the actual extremes of the flux variation
at these frequencies. Even the relatively sparse cm-data includes data
points from observations taken several years apart and thus can be
taken as an indication of approximately the true minima and maxima,
giving the spectrum a convex shape at all times.

\paragraph{B1349$-$439.} 
% BLO
The source identification has been discussed in detail in
\citet{veron96}. The paper shows that there are two optical sources
near the radio source PKS~1349$-$439. PKS~1349$-$439 can be identified
as a BL Lac object with a high and variable optical polarization
($p_{max}=$~21.5\%, \citet{impey88}), and the other one is a radio quiet
Seyfert 1 galaxy at $z=$~0.052. \citet{veron96} call the source a flat
spectrum radio source, but our plot, with added low-frequency
varaibility data from HartRAO and high-frequency SEST data, shows that
the spectrum is inverted up to 8 GHz.
% Beasley data lis�tty omaan taulukkoon.

\paragraph{B1954$-$388.} 
% HPQ
($z=$~0.626)
% z: SIMBAD, ref. 1997MNRAS.284...85D
An HPQ with $p=$~10.9\% \citep{impey90}.
This source has an inverted spectrum up to around 10 GHz. The number
of 2 and 5 GHz observations is larger than that at the other low 
frequencies, the flux density at 2 GHz varying between 1 and 2 Jy.
The catalog values for 640 MHz and 1.4 are higher than
the minima at 2 GHz, but the observations are also non-simultaneous.
The variability at 90 GHz is relatively strong, and the flare spectrum
from approximately mid-1992 (taken at 2.3, 4.8, 8.4, 90 and 230 GHz) 
is strongly inverted up to 90 GHz.

\paragraph{B2128$-$123.} 
% LPQ
($z=$~0.499) An LPQ ($p_{max}=$~0.6\%, 
\citet{stickel94}) with an inverted spectrum up to ca. 5 GHz and 
evident variability especially above 8 GHz.

\paragraph{B2136+141.} 
% LPQ
($z=$~2.427) An LPQ ($p_{max}=$~1.6\%,
\citet{stickel94}) with a spectrum that is slightly inverted up to 
8--10 GHz in the quiescent state, and clearly inverted 
($\alpha\geq +0.5$) during outbursts.

\paragraph{B2254$-$204.} 
% BLO
A BLO with high optical polarization ($p=$~14.3\%,
\citet{impey88}). The source is one of the relatively rarely observed
radio sources, the spectral plot only contains radio data from PKS and
our own observations (HartRAO and SEST).  The current data show a
spectrum that is inverted up to ca. 15--20 GHz.
% Ohio OY 290 S(1.415)=0.21 Jy

\paragraph{B2255$-$282.} 
% LPQ
($z=$~0.926) An LPQ ($p=$2\%, \citet{impey90})
that was the subject of a multifrequncy study in \citet{tornikoski99a}.
In 1997 this source was observed in a very active state
in the millimeter domain, and later it was found to have a very bright
high-energy $\gamma$-ray outburst, observed by the EGRET. The source
has exhibited blazar-like variability in its active state, during which
no optical polarization measurements were unfortunately made.
\citet{tornikoski99a} noted that the radio spectrum in its
quiescent state peaks above 10 GHz, and during the millimeter flare
at around 90 GHz. In Fig.~2f we reproduce the data plot with some additional
post-flare data points.

%-----

\subsection{Other sources with inverted spectra}
\label{subsec:cand}

In Fig.~\ref{fig:inv1} and Fig.~\ref{fig:inv2}
we have plotted the radio spectra of all of the sources that have
a somewhat inverted spectrum, but most of which show highly inverted
spectra only during flares and are otherwise relatively flat, or which
lack data to determine the true shape of the spectrum. The data 
for these plots are as 
listed in  Table~\ref{tab:datainfo}.

\paragraph{B0252$-$549.} 
% QSO
($z=$~0.537) A rarely observed source with no polarization information
available. Our spectrum consists of only 7 data points, taken at 6
different frequencies. Thus no variability information is available
and the frequency coverage is sparse.  The plot shows a spectrum that
is moderately inverted from 2.7 to 90 GHz, but the low-frequency part of the
spectrum is flat rather than inverted, and the inverted shape of the
high-frequency part may be due to variability only.

\paragraph{B0402$-$362.} 
% LPQ
($z=$~1.417) An LPQ ($p=$~0.6\%, \citet{impey90})
% Sandqvist A et al. 1995A&A...295..585
% k�ytt�neet kalibrointil�hteen�: 4.174 Jy @�2cm, 1.033 Jy @ 20 cm, 1983
that shows very strong variability in the millimeter-domain
but for which no low-frequency variability information have
been published. Our plot shows a spectrum that is initially flat
but strongly inverted from 2 to 8 GHz. The strong variability in the
millimeter domain leads us to suspect that the true quiescent spectrum
of this source might be flat, and get inverted only during flares.
% Huom. todellisuudessa oikeastaan hyvin samanlainen kuin 1444-379!
 
\paragraph{B0539$-$057.} 
% HPQ
($z=$~0.839) This source is an HPQ with 
$p=$~4.5\% \citep{stickel94}. The variabilty apparent in the
8 GHz data and the lack of variability information at the other
frequencies prevent us from determing the exact peak frequency,
with the current data it can only be estimated to be between
5 and 20 GHz.

\paragraph{B1145$-$071.} 
% QSO
($z=$~1.342) This source was among the first discovered real binary
quasars, and the very first binary where one quasar is radio bright,
and the other one is radio faint \citep{djorgovski87}.  Our radio
spectrum is inverted up to 5 GHz.  The source exhibits 
variability at least in the cm region.  The 5 GHz data were obtained
during two epochs only, whereas the 8 GHz data set spans over 18 years
of time, and thus the flux minima at 5 GHz may not represent the true
quiescent state. The source probably is a variable source with a flat
spectrum that gets inverted during outbursts.

\paragraph{B1546+027.} 
% HPQ
($z=$~0.413) An HPQ with strong and rapid polarization variations
($p_{max}=$~13\%, \citet{smith94}) and also in the radio domain a highly
variable source with an inverted spectrum up to the GHz region
in the quiescent
state, and up to at least 20 GHz in the flaring state. The high
frequency part of the radio spectrum remains extremely flat also in
the quiescent state ($\alpha\approx -0.3$).

\paragraph{B2145+067.}
% LPQ
($z=$~0.990) An LPQ with $p=$~1.03 \citep{wills92}.
The spectral shape is flat up to 5 GHz, after which it is inverted up
to ca. 20 GHz. The source exhibits very strong variability, but the
spectral shape after 5 GHz remains constant from the low to the high state.  

\paragraph{B2216$-$038.}
% LPQ
($z=$~0.901) An LPQ ($p_{max}=$~1.2\%, 
\citet{stickel94}) with an inverted spectrum between 1 and 8 GHz,
and variablity observed across the radio spectrum. 

\paragraph{B2243$-$123.} 
% HPQ
($z=$~0.630) An HPQ ($p_{max}=$~5\%, \citet{stickel94}) with a slightly
inverted spectrum up to 5 GHz. If the best sampled 8 GHz monitoring
data, 61 data points spanning from 1981 to 1999, represent the true
variability also at the other, much more sparsely sampled,
frequencies, this source is only a flat-spectrum source with
relatively high variability.

In addition to the aforementioned sources with inverted spectra,
there are several sources with very flat spectra that get inverted
during outbursts: \objectname[]{B0109+224}, \objectname[]{B0306+102},
\objectname[]{B0754+100}, \objectname[]{B0823+033},
\objectname[]{B1144$-$379}, \objectname[]{B1244$-$255},
\objectname[]{B2121+053}.  When studying the flare spectra only, or in
many cases even the average spectra, these spectra show convex,
GPS-like shapes. But our data base with years of variability data
shows that calling these sources GPS sources would be an
over-interpretation based on too sparse data. The variability issue is
addressed in more detail in Section~\ref{sec:variability}.

%----------

\section{Variability of GPS sources}
\label{sec:variability}

A common feature for almost all of the sources introduced in
Section~\ref{sec:candidates} is strong variabilty across the radio
spectrum and especially in the millimeter domain.

Earlier it has been argued that GPS-type sources have very low radio
variability. On the other hand, the high radio-frequency variability
of GPS sources has not been studied very much. In
\cite{tornikoski2000} we showed that the millimeter domain variability
of the two bona fide GPS sources, \objectname[]{B0238$-$084} and
\objectname[]{B0743$-$006} is about as strong as that of the other
typical flat-spectrum quasars.  The two newly identified GPS sources,
\objectname[]{B1936$-$155} and \objectname[]{B2008$-$159}, also showed
strong variability.  Variability of some other GPS sources has also
been confirmed by \citet{edge96, edge98}. It has been proposed,
however, that the possible variabilty in GPS sources should be
coherent across the spectrum, whereas the
variability in ordinary flat-spectrum sources, caused by components
peaking at different frequencies, is likely not to occur coherently
across the whole spectrum \citep{snellen95, snellen98b}.

Because at least some of the GPS sources seem to exhibit
such strong variability in the millimeter domain, we also wanted to
study the variability of the known GPS sources, and also to see
whether variable sources may have been
misidentified as GPS sources when only sprasely sampled spectra have
been available for the identification.  For example the source
\objectname[]{B0109+224} from our original SEST sample
has a spectrum that is completely flat up to
230 GHz in the quiescent state, and highly inverted ($\alpha\geq
+0.5$) up to 40 GHz in the flaring state. If only a very limited number
of data points are available, this kind of a highly variable source is
easily (mis-)classified as a GPS source. As mentioned at the end
of Section~\ref{sec:candidates}, our sample consists of several
sources with spectra similar to that of \objectname[]{B0109+224},
i.e. flat in the quiescent state, and inverted during outbursts.

In Table~\ref{tab:gps_info} we list all of the known GPS sources
(including some GPS source candidates) that have been observed at SEST
by our group, together with their flux information at 90 and 230 GHz.
In the following we discuss their radio spectra in more detail,
excluding the sources already discussed in \citet{tornikoski2000} and
the CSS source \objectname[]{B1151$-$348}.  The radio spectra of the
sources discussed here are plotted in Fig.~\ref{fig:bfgps1} and
Fig.~\ref{fig:bfgps2}.

\paragraph{B0440$-$003 = NRAO~190.}
% ``GPS?''
($z=$~0.844) An HPQ ($p_{max}=$~12.5\%, \citet{stickel94}) that is
identified as a GPS candidate in \citet{stanghellini98}, with $\nu_T$=0.3 GHz. 
Our plot shows a highly variable spectrum peaking at $\sim$2 GHz.

\paragraph{B0511$-$220}
% GPS
($z=$~1.296) A quasar with an unknown optical polarization that
is listed as a GPS source candidate in \citet{dallacasa98}, with 
$\nu_T\approx$5 GHz. Our data confirm the GPS-shape of the spectrum.

\paragraph{B0528$-$250.}
% ``Not a GPS''
($z=$~2.779) An LPQ ($p_{max}=$~0.7\%, \citet{stickel94}). 
This source is listed as a GPS source in \citet{devries97} and
\citet{odea98}, with $\nu_T$=1.4 GHz. 
Our SEST observations have all been non-detections ($S/N<4$),
% Coord OK!
in Table~\ref{tab:gps_info} the upper limits only are given for this source
at 90 and 230 GHz.  This indicates that the optically thin part of the
spectrum falls very steeply. Our data plot for this source does not
reveal any truly inverted shape in the spectrum above 1 GHz, and since
the low-frequency part of the spectrum seems to be flat and variable,
we suggest that this source should not be classified as a GPS source.

\paragraph{B0528+134.}
%  ``inverted during flares'' 
($z=$~2.067) An LPQ ($p_{max}=$~0\%, \citet{stickel94})
that was identified as a GPS candidate in
\citet{stanghellini98}, with $\nu_T$=1.9 GHz.  
Due to its status as a flaring gamma-ray
source \citep{hunter93, mukherjee96, boettcher98, zhang98, mukherjee99} 
this source has been a target of several multifrequency
campaigns and it has been under extensive flux monitoring
during the recent years. It shows high variability in the
cm- and mm-domains. If we study the quiescent spectrum, i.e. the
minimum flux densities at each frequency, we notice that the
spectrum is very flat all the way to ca. 40 GHz, but inverted
only between 0.4 and 1 GHz, where only very limited number of data
points are available.
During outbursts the spectrum is extremely inverted.

\paragraph{B1127$-$145.} 
% Not a GPS
($z=$~1.187) An LPQ with $p_{max}=$~1.2\% \citep{stickel94}.  This
source has been listed as a GPS source in \citet{stanghellini98} and
\citet{odea98}, with $\nu_T$=1 GHz. 
Our spectral plot shows a falling spectrum after 1.4
GHz, with notable variability.  The data points taken at HartRAO in
1991--1992 at 2.3 and 4.8 GHz show much lower flux densities than the
values given in PKS and K1.  The relatively high state of the PKS and K1
data probably explains why this source has been classified as a GPS
source. We believe that also the 1.4 GHz PKS and K1 data points show
the spectrum in a flaring state, which means that the general spectral
shape after 1 GHz is falling and the source should not be classified
as a GPS source.

\paragraph{B1519$-$273.}
% ``inverted during flares'' 
A BLO with unknown redshift and high optical polarization
($p_{max}=$~11.4\%, \citet{impey88, impey90}). This source is listed
as a bona fide GPS source in \citet{devries95}, with $\nu_T$=5 GHz. 
Our plot shows a relatively flat quiescent spectrum that is inverted during 
flares.

\paragraph{B1718$-$649.} 
% GPS
($z=$~0.014) This source was identified as the nearest GHz
peaked-spectrum radio galaxy in \citet{tingay97}, with $\nu_T$=3 GHz.
Some variability is observed in the hig-frequency flux measurements,
but with the current data sets the spectrum has a characteristic convex
shape, peaking near 3 GHz.  The VLBI observations in
\citet{tingay97} also confirm the GPS nature of this source.

\paragraph{B2126$-$158.} 
% Not a GPS
($z=$~3.266) An LPQ with $p_{max}=$~1.5\% \citep{stickel94}. This
source was classified as a GPS source in \citet{devries97,
stanghellini98, odea98}, with $\nu_T$=4 GHz. Our plot shows a
flat spectrum that steepens after 8 GHz in the quiescent state
but remains flatter in the flaring state. The quoted $\nu_T$=4 GHz
is probably an over-interpretation based on only PKS and K1
catalog data.

\paragraph{B2134+004.}
% GPS
($z=$~1.932) An LPQ with $p_{max}=$~2.3\% \citep{stickel94}. 
This source has been identified as a GPS
source in \citet{stanghellini98} and \citet{odea98},
with $\nu_T\approx$5 GHz. Our plot shows
an extremely beautiful example of a convex spectrum, peaking
between 5 and 8 GHz. The source exhibits some variability, but
the spectral shape remains an extraodrinarily pronounced GHz 
peaked-spectrum in spite of the variations.

\paragraph{B2230+114, CTA~102.}
% Not a GPS
($z=1.037$) An HPQ with $p_{max}=$~10.9\% \citep{stickel94}.  This
source has been classified as a GPS source candidate in
\citet{stanghellini98}, with $\nu_T$=0.5 GHz.  In our plot the
quiescent spectrum is steadily falling (with $\alpha\approx -0.5$)
after 0.4 GHz. This source is well known for its rapid and strong
radio variability, sometimes connected to optical variations
\citep{tornikoski99b}, and the extremely high variability is evident
from our data plot also. The 327 MHz data points from \citet{ghosh94}
confirm also the low-frequency domain variability, leading us to
suggest that the steepening seen between 180 and 410 MHz is due to
sparsely sampled, non-simultaneous data instead of the
actual average/quiescent spectrum of the source.

\section{Discussion}
\label{sec:discussion}

We have presented radio spectra for 12 sources that we call
new quasar-type GPS source candidates, having quiescent spectra with an 
inverted
shape, peaking at frequencies $\nu_T>$1 GHz. 6 of these sources
have extremely high peak frequencies, $\nu_T\geq$10 GHz in the 
observer's frame.
In addition to these 12 sources we have presented a set of 8 other
sources with inverted parts in their spectra, but either 
having too flat spectra for actual GPS sources or being
too poorly sampled to determine whether they are
GPS candidates. All of the 12+8 sources for which we have 
multi-epoch data exhibit high variability.

We calculated the spectral index from 90 to 230 GHz for our 12 GPS
candidate sources. For the two sources for which either the 90 GHz or 
the 230 GHz data were unavailable, we used the 150 GHz data point from the
IRAM data instead.  The median spectral index between 90 and 230 GHz for both
the quiescent as well as the average spectra of these sources is
$\alpha=-0.63$, the quiescent spectral indices ranging from
$\alpha=-0.20$ to $\alpha=-1.17$. The ``generic'' high-frequency
spectral index presented by \citet{devries97} for GPS sources was
$\alpha=-0.73$, only a little steeper than our sample median.  For the
complete sample of southern AGNs in \citet{tornikoski2000} the median
quiescent spectral index between 90 and 230 GHz was $\alpha=-0.74$,
i.e. not greatly different from that of the present sample.  During
flares the turnonver frequencies can be shifted to as high as 100 GHz,
but for most of the sources the 100--230 GHz spectral indices remain
almost constant from the quiescent to the flaring state, the median 90
to 230 GHz spectral index in the flaring state being $\alpha=-0.60$.

The redshifts of the new GPS candidate sources range from 0.5 to
2.4, with only one having a redshift of $z=$2.4, and the others having
redshifts of $z<$1.5. There is no redshift information available
for 4 sources.  6 out of the 12 GPS candidates are reported to have optical
polarizations $p>$3\%, some as high as $p>$20\%, 3 sources have a low
optical polarization $p<$3\%, and for additional 3 sources no optical
polarization information was available. There are no significant
correlations between the turnover frequency, optical polarization,
redshift and the spectral steepness.
The variability of these sources does not seem to directly
correlate with the turnover frequency or any other property, but the
correlation is difficult to verify because the data sets are
very inhomogeneous (different amounts of data, different frequency
coverages, non-simultaneous data taking, etc.)

In addition to the strong variability, many of our sources also show
other characteristics usually considered to be more typical to the
blazar-type sources: high optical polarization, strong and variable
multiwavelength emission, high-energy $\gamma$-ray flares, etc.

Our group has also made millimeter observations of a sample of 14
sources identified as GPS sources in the literature.  Many of these
``bona fide'' GPS sources are highly variable, and several of them
seem to be variable, flat-spectrum sources that have been
misidentified as GPS sources due to too sparse data sampling.  In
\citet{marecki99} the authors argue that because most of the known GPS
sources show hardly any variability, sources that show variability
should probably not be called GPS sources.  Our data show that sources
with persistent GPS-type spectra at all stages of activity (e.g.,
B0238$-$084 and B0743$-$006, presented in \citet{tornikoski2000}, and
0511$-$220 and B2134+004, discussed in Section~\ref{sec:variability})
can be variable up to the millimeter domain. In \citet{odea98} the
linear sizes of B0743$-$006 and B2134+004 are given as 0.036 kpc and
0.017 kpc, respectively, which shows that these sources are also
extremely compact and thus ``typical'' GPS sources.  Even though we
have shown the risks of using sparsely sampled data to make GPS
identifications, we also suggest that at least some of the quasar-type
GPS sources are indeed highly variable.

In the light of our data sets it seems evident that there are
two main classes of peaked-spectrum qusars: 1) the variable
flat-spectrum sources that have a clearly inverted spectrum only during
outbursts; and 2) the variable inverted-spectrum sources whose
spectral shapes remain inverted at all stages of activity and which
thus adhere to the definition of the genuine GPS sources.
The present complete samples of GPS/CSS sources
should be redefined to avoid the
inclusion of sources where several components (e.g., core, jet,
consecutive shocks) may contribute to the shape of the spectrum.

In our present sample there are no non-variable quasar-type
peaked-spectrum sources, but this may be due to the selection effect
of having only studied sources that had been included in our
monitoring campaigns, known or supposed to be variable and thus
``interesting''.  However, there have been no systematic long-term
studies of the radio-to-millimeter variability of the complete sample
of quasar-type GPS sources, so it is possible that they all
can exhibit moderate to high variability. Dedicated
variability studies across the radio spectrum are needed in order to
determine whether variability actually is typical for the quasar-type
GPS sources, as well as to determine which sources are variable,
flat-spectrum sources showing inverted spectra only at some stage of
activity or when only non-simultaneous data are available, and
which have gigahertz-peaked spectra at all times.

Several sources in our sample have high peak frequencies.
Even though sources peaking anywhere above 1 GHz are by the classical
definition GPS sources, it is possible that these moderate-redshift,
bright, variable sources with spectral peaks well above
1 GHz (many even above 10 GHz in the observer's frame, 20--40 GHz in
the rest frame) are fundamentally very different from the ``original''
GPS sources.
Even though the maximum frequency of the spectral peaks of the known GPS
sources to date seems to be around 20 GHz in the observer's frame, we
can not rule out the possibility that even higher peak frequencies
could exist. Both VLBI and IR/optical images as well as multifrequency
variability data will be needed to determine whether the low-peaked,
high-peaked, and possible ultra-extreme-peaked GPS sources belong to
the same class of objects, maybe presenting a continuity of properties
from one extreme to the other, or whether the low-peaked and
high-peaked sources form two (or more) very different classes of
objects.

Because most high-frequency radio observing campaigns are based
on selections made at low radio-frequencies, the actual number of
extreme-peaked quasars may be much higher than currently
known. Sources that have catalog data points only fainter than 
0.5 Jy at 2.3 or 5 GHz have extremely rarely been observed 
at the higher frequencies. For extreme peak-frequencies
in the 20--40 GHz range, fainter sources with $S<$0.5 Jy at 5 GHz
could show flux levels as high as 1--1.5 Jy at the peak-frequency,
assuming the canonical GPS spectrum, and still much higher, assuming
a more steeply rising spectrum like the one
predicted by the homogenous self-absorbed synchrotron source model.
It is important to investigate whether there is such a population
of undetected extreme-peaking sources faint in the low-frequency domain.
Our group is in the process of conducting a study
of such fainter sources.

\section{Conclusions}

We have presented a set of radio spectra based on our group's long
term high radio-frequency (90 and 230 GHz) observations with the SEST,
complemented with data from the literature. We have identified 12
quasar-type sources with convex spectra resembling those of the GPS
sources and peaking at high frequencies. 6/12, i.e. 50\%, of the
sources have their spectral turnover at $\nu_T\geq10$ GHz in the
observer's frame, which makes them unusually high-peaked sources.
We also present 8 additional sources with inverted spectra in the GHz
region but either with too little data to make conclusive statement
about their true spectral shape or with spectral shapes, though at
least somewhat inverted, not strictly adhering to the generic GPS
shape.  Probably at least some of these sources are only flat-spectrum
sources that are variable and for which the often sparsely sampled
data were taken during a relatively active state.

It is not clear whether the low-peaked and high-peaked GPS sources
belong to the same class of objects, maybe presenting a continuity of
properties from one extreme to the other, or whether they form two
very different classes of objects, showing only similar spectral
shapes. It is possible that the number of these extreme
peaked-spectrum sources is much higher than earlier assumed, and that
the small number of currently known such sources is mainly due to the
fact that no search for them has been conducted at high
radio-frequencies.  To study these sources in more detail and to
determine the true number of such sources one needs data sets with
dense temporal sampling to exclude the sources with spectra flattened
by an outburst.

The variability of the new high-peaked sources as well as that of 
the earlier known GPS sources observed by our group is strong, thus the
often quoted statement that GPS sources would belong to the least
variable class of compact extragalactic radio sources seems to be
incorrect for these qusar-type GPS sources.
On the other hand, the strong variability of flat-spectrum radio sources
may also lead to misidentifications. We have shown 6
cases where the densely sampled radio spectrum of the sources quoted
as GPS sources in the literature is in conflict with the generally accepted
definition of a GPS source spectrum.  If the variable sources are observed only
few times, the source may have been in an active state during the
observation and showing an unusually flat, or even inverted,
spectrum. With dense long-term monitoring it is possible to observe
the actual flux density minima and maxima, and thus determine the
overall shape of the source spectrum.

\acknowledgments
The SEST is operated jointly by ESO and the Swedish National Facility for 
Radio Astronomy, Onsala Space Observatory at Chalmers University of
Technology. MT acknowledges the support of the Academy of Finland
for our SEST observing projects.
This research has made use of data from the University 
of Michigan Radio Astronomy Observatory which is supported by funds from the
University of Michigan.
This research has also made use of the SIMBAD database, operated at CDS,
Strasbourg, France, and the NASA/IPAC Extragalactic Database (NED) 
which is operated by the Jet Propulsion Laboratory, California Institute 
of Technology, under contract with the National Aeronautics and Space
Administration. 

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\newpage

\figcaption[Tornikoski.fig1.eps] % Figure 1
{New sources with GPS-type spectra. 
a) B0048$-$097, b) B0332$-$403, c) B0454$-$234, d) B0537$-$441, 
e) B1057$-$797, f) B1334$-$127.
\label{fig:newgps1}}

\figcaption[Tornikoski.fig2.eps] % Figure 2
{New sources with GPS-type spectra, continued.
a) B1349$-$439, b) B1954$-$388, c) B2128$-$123, d) B2136+141,
e) B2254$-$204, f) B2255$-$282.
\label{fig:newgps2}}

\figcaption[Tornikoski.fig3.eps] % Figure 3
{Other sources with inverted spectra.
a) B0252$-$549, b) B0402$-$362, c) B0539$-$057, d) B1145$-$071.
\label{fig:inv1}}

\figcaption[Tornikoski.fig4.eps] % Figure 4
{Other sources with inverted spectra, continued.
a) B1546+027, b) B2145+067, c) B2216$-$038, d) B2243$-$123.
\label{fig:inv2}}

\figcaption[Tornikoski.fig5.eps] % Figure 5
{Radio spectra of bona fide GPS sources and GPS candidates.
a) B0440$-$003, b) B0511$-$220, c) B0528$-$250, d) B0528+134, 
e) B1127$-$145, f) B1519$-$273.
\label{fig:bfgps1}}

\figcaption[Tornikoski.fig6.eps] % Figure 6
{Radio spectra of bona fide GPS sources, continued.
a) B1718$-$649, b) B2126$-$158, c) B2134+004, d) B2230+114.
\label{fig:bfgps2}}

% tables:
\input{Tornikoski.tab1} %tab:datainfo
\input{Tornikoski.tab2} %tab:cand_gps_info
\input{Tornikoski.tab3} %tab:cand_inv_info
\input{Tornikoski.tab4} %tab:gps_info

\end{document}
