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\received{}
\accepted{}
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\slugcomment{Submitted to the Astrophysical Journal: 
April 22, 2002;\\
Revised June 25, 2002}

\shorttitle{Possible Identifications for Southern EGRET Sources}
\shortauthors{Tornikoski et al.}

\begin{document}

\title{
  Possible Identifications for Southern EGRET Sources}

\author{M. Tornikoski\altaffilmark{1} and
        A. L\"ahteenm\"aki}
\affil{Mets\"ahovi Radio Observatory, Mets\"ahovintie 114, FIN--02540
         Kylm\"al\"a, Finland}
\author{M. Lainela}
\affil{V\"ais\"al\"a Institute of Space Physics and Astronomy,
         Tuorla Observatory, FIN--21500 Piikki\"o, Finland}
\author{E. Valtaoja}
\affil{V\"ais\"al\"a Institute of Space Physics and Astronomy,
         Tuorla Observatory, FIN--21500 Piikki\"o, Finland\\
         and: Dept. of Physical Sciences, University of Turku,
   FIN--20100 Turku, Finland}

\altaffiltext{1}{
  Email: Merja.Tornikoski@Hut.Fi}

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

\begin{abstract}
We have made total flux density observations at high radio frequencies
(90 and 230 GHz) of 12 Southern AGNs that were classified as possible
ERGET identifications in the Third EGRET Catalog.
Our observations confirm the blazar-nature of five of them.
We have also studied sources
that we considered good candidates for AGN counterparts of 
previously unidentified EGRET sources and which had not been observed in the
millimeter-domain before. Four of them showed millimeter-range activity 
which may be related to their gamma-ray activity,
making them good candidates for the EGRET source
identification.
\end{abstract}

\keywords{galaxies: active -- gamma-rays: observations -- 
quasars: general -- radio continuum: galaxies}


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


\section{Introduction}

The third catalog of high-energy gamma-ray sources detected by EGRET
\citep{hartman99}, hereafter 3EG, includes 271 sources.
66 of them are high-confidence
identifications of blazar-type active galactic nuclei (AGNs):
BL Lac objects, flat-spectrum radio qusars, or unidentified flat-spectrum
radio sources, and 27 of them are
lower-confidence potential blazar identifications. 170 sources (96 of
which have $|b| > 10^{\circ}$) in 3EG were not identified firmly with known
objects.

The identification of the EGRET source counterparts is difficult
because the source of the gamma-ray emission is not well located.
The sources classified as possible AGN identifications in 3EG are
questionable, either because the objects have low radio flux density
(5 GHz data from the literature with $S<300$ mJy) or because they lie outside
the $95\%$ uncertainty contour, sometimes even outside the $99\%$
contour.

% 1 EG;n AGN-identifikaatiot peraisin 
% Kuehr 1981
% Burbidge & Hewitt 1987 (<-- ei radiodataa)
% Impey & Neugebauer 1988 (FIR dataa), 
% Stickel 1991 (k�ytt�� Kuhrin radiodataa), 
% Valtaoja 1988 (oikeasti viittaa Kurp dataan vuosilta 1978-1985) 
% Waltman 1991 ( 2.7 and 8.1 GHz - 1979-1987)
% Wiren 1992 (huhti-88 - touko-89)
% Zensus 1989 (kokousjulk. supeluminaalisista, ehkei flukseja? listataan silti)
% Wills et al. 1992 (optista), Webb 1988 (optista) 
% Impey & Tapia 1990 (opt. pol.), Sitko & Sitko 1991 (NIR)
%  Brown 1989 
% Lin, 5th C. Symp. s. 347: 0.3 Jy at 5 GHz.
% ** Mattox 2001: PMN catalogs Wright et al. 1994, Griffith et al. 1994, 1995
%                               June 1990
%                                                  During 1990
%                                                                    1990-Nov

Because so many EGRET-detected sources have been identified with
blazars, it is a natural hypothesis that some of the unidentified EGRET
sources are also blazars. Several earlier papers have been published
that present methods to try to identify unidentified EGRET sources 
with AGNs \citep[e.g.,][]{mattox97, punsly97, bloom97, zook97, combi99, 
mattox01}.

We had two major goals in this project: we wanted to make further
studies of probable/potential candidates already suggested in the
literature, and we also wanted to propose possible new candidates for
the unidentified sources listed in 3EG and to study a sample of those
candidates to see if our method of using millimeter-wavelength
observations is feasible for making new identifications.

In Section~\ref{sec:obs} we describe our observations.  In
Section~\ref{sec:back} we discuss the method of identifying EGRET
sources using millimeter-domain data.  In Section~\ref{sec:lowconf} we
present our results for 12 Southern sources classified as potential
blazar identifications in 3EG, in Section~\ref{sec:msm-cand} we
present the results for some sources suggested for the EGRET source
counterparts earlier in the literature, and in
Section~\ref{sec:newcand} we present the results for a set of sources
we identified as possible counterparts for the previously unidentified
EGRET sources.  The conclusions are given in
Section~\ref{sec:conclusions}.


\section{Millimeter observations}
\label{sec:obs}

The aim of this project was to study the high radio-frequency
data of the potential EGRET counterpart AGNs in order to see
if they are bright and variable, with flat or even inverted
high-frequency part of the radio spectrum, and thus good
candidates for the EGRET identifications.
For the sources with long data sets we also wanted to study
if they were going through a flaring stage at high radio frequencies
at the time of the EGRET-detection.

Our group has been studying AGNs at high radio-frequencies since 1987
with the 15-meter Swedish--ESO Submillimeter Telescope (SEST) at the
European Southern Observatory (ESO) site of La Silla, Chile, latitude
$29\arcdeg$ South. The observing frequencies were 90 GHz (3 mm) and
230 GHz (1.3 mm). Originally, the observations were made as described in 
\citet{tornikoski96}, but since June 1994 an SIS mixer
receiver was used for the 90 GHz observations instead of the Schottky
receiver.  The bolometer was used for the majority of the 230 GHz
observations, resulting in improved sensitivity compared to the
Schottky/AOS setup used earlier.

During our various observing projects with the SEST we have observed
both well-known strong and variable sources as well as some samples of
rarely observed sources \citep[e.g.,][]{tornikoski93, tornikoski99,
tornikoski00b, tornikoski01}; details about the SEST AGN monitoring
observations can be found in \citet{tornikoski96} and Tornikoski (2002,
in preparation). For
some of the objects studied in this paper (many of the sources studied
in Section~\ref{sec:lowconf}) we have millimeter-wave monitoring data
spanning a decade, giving us a very good estimate of their variability
behavior over a very long period of time. Most of the objects studied
in Section~\ref{sec:msm-cand} and Section~\ref{sec:newcand} had never been
observed in the millimeter-domain before, and for this paper we observed them
at 90 GHz in April 2000 and August 2000.

For the plots and tables in this paper our SEST data sets were
complemented with multifrequency radio data from the literature.  These
data are from \citet{kuehr81, lawrence83, salonen87, terasranta87, steppe88,
wright90, large91, wright91, terasranta92, steppe92, steppe93, tornikoski93,
bramwell93, ghosh94, bondi96, beasley97, bloom97, drinkwater97, zook97, reuter,
reich98, terasranta98, kovalev99, reich00, terasranta01}. 


\section{Background}
\label{sec:back}

Practically all EGRET-detected AGNs identified so far are
bright and variable in the millimeter range, but all radio-bright
and variable AGNs have not been detected by EGRET. 
Our earlier work \citep{valtaoja96, lahteenmaki97, tornikoski99,
lahteenmaki00, tornikoski00a} has shown that
there seems to be a connection between the high radio-frequency and
EGRET gamma-ray activity, and that the most probable blazar to be
detected with EGRET is a source with an ongoing and still rising
high-frequency radio flare.

The case of \objectname[]{PKS\,2255$-$282} 
\citep{tornikoski99} demonstrates that
an AGN can remain undetected by EGRET until it goes through an activity
period which is observed both in the radio domain and at the gamma-ray
energies. Thus it is highly possible that several of the unidentified
EGRET sources in 3EG will turn out to be radio loud AGNs which
probably would have shown also millimeter-wavelength activity at the time
of the EGRET detection.

To summarize the millimeter-properties of the known EGRET blazars, we
have collected in Table~\ref{tab:highconf} the 5 GHz, 22 GHz and 90
GHz data for all of the EGRET-detected sources in 3EG with a
high-confidence identification with an AGN (marked as ``A'', an AGN, in
3EG). The table lists the flux density minima and maxima as well as
the number of data points at each frequency band, respectively.  For
the sources with at least one good-quality (signal-to-noise-ratio $>$
4) data point at two of these frequencies, we have calculated the
spectral index $\alpha$ (using $S\propto\nu^\alpha$).
Many of these sources have been standard targets
of our AGN monitoring projects with the SEST telescope at 90 GHz,
and as shown in the table, for many of these sources we have as many
as over a hundred data points. Because especially the 90 GHz fluxes of 
these sources can radically vary between the quiet and the active
states, we have calculated two different spectral indices:
the $\alpha_{quiet}$ is calculated using the minimum flux densities
at both 5 and 90 GHz, illustrating the shape of the assumed quiescent 
spectrum, and the $\alpha_{active}$ is calculated using the maximum
flux densities at 5 and 90 GHz, illustrating the shape of the spectrum
during the active state. Between 5 and 22 GHz
as well as 22 and 90 GHz only the $\alpha_{active}$ is shown.
Since exactly simultaneous data do not exist for most of the sources,
we chose to use the historical maxima at each frequency to calculate
the $\alpha_{active}$. It is possible that the radio outburst
development in reality is such that the maxima at lower frequencies
are only reached after the flux is already decreasing at the higher
frequencies, which means that the actual shape of the spectrum during
the millimeter-wavelength activity peak is even flatter than indicated
by the $\alpha_{active}$ index, but this practise at least prevents us
from overestimating the flatness of the spectra.

As can be seen from Table~\ref{tab:highconf} and from the spectral
index distribution plot Fig.~\ref{fig1}, the EGRET-detected
blazars have extremely flat 5 to 90 GHz spectra especially in the
active state. The median $\alpha_{5,90;active}$ during
millimeter-domain activity is $\alpha=+0.02$ (for the ones that we
have data for at 90 GHz). 
For comparison, if we only study the sources considered as
``EGRET identifications with a high probability of being correct'' 
in \citet{mattox01}, we get the median spectral index 
$\alpha_{5,90;active}=+0.09$.

If all EGRET-detected AGNs have flat spectra up to the millimeter range
and are variable, it is reasonable to assume that in order to be a
possible counterpart for a so far unidentified EGRET source, the AGN
has to have a flat radio-to-millimeter spectrum and be variable 
in the high-frequency radio domain.

We started by investigating the southern sources classified as
potential EGRET counterparts in 3EG (marked as ``a'', a possible AGN,
in 3EG).  In this paper we study their full radio spectra and the
long-term behavior at 90 and 230 GHz in order to determine if their
activity in the millimeter region could confirm the identification as
a blazar counterpart for the EGRET detection.

We also made high radio-frequency observations of the previously
unidentified EGRET sources that could have a blazar counterpart.  We
study a set of sources relatively faint at 5 GHz (catalog data), and
thus routinely excluded from any high radio-frequency studies, to
search for AGNs with unusually flat spectra. Our assumption was that
if we find objects faint at 5 GHz having a spectrum that is extremely
flat or even inverted up to the millimeter-domain, these are probably
sources extremely variable at millimeter wavelengths and also good
candidates for gamma-ray active blazars.

\section{Lower-confidence AGN identifications in 3EG}
\label{sec:lowconf}

3EG lists thirteen Southern sources which are potentially identified
with blazars. These sources are listed in Table~\ref{tab:lowconf}
and their spectral index distribution is plotted in Fig.~\ref{fig2}. 
The identifications are
considered to be of low confidence either because the object has a low
radio flux reported in the literature (historical 5 GHz flux density
$S < 0.3$ Jy) or because it lies outside the 95\% uncertainty contour.

If all the lower-confidence identifications with AGNs are grouped
together with the high-confidence identifications and studied
as ``EGRET detected blazars'', incorrect identifications will easily lead
to incorrect conclusions about the source properties.
Thus it is important to try to determine
whether the possible identifications given in 3EG are correct or not.

\citet{mattox01} have studied the possible source identifications
with an AGN in 3EG by calculating the 
probability of identification using the method described in
\citet{mattox97}. The method is based on using the radio fluxes
at 5 GHz. The conclusion of their analysis is that 45 out of the 66
``high-confidence AGN identifications'' in 3EG have a high probability
of being correct and 20 of them are plausible. Of the 27 ``lower-confidence 
identifications'' in 3EG they only find one to be robust,
three to be plausible, and 23 unlikely.

There are several problems with trying to identify a promising AGN
counterpart using low-frequency radio data from published catalogs. The
5 GHz catalog data used for the 3EG source identifications of the Southern
sources \citep{wright94, griffith94, griffith95} are from 1990,
i.e. nonsimultaneous with the EGRET observations.  As mentioned in
Section~\ref{sec:back}, there is plenty of evidence that the gamma-ray
activity of AGNs correlates with the millimeter-domain activity. Many
AGNs, however, are known to be highly variable in the radio/mm domain,
and thus the historical one-epoch low-frequency fluxes can not be
expected to correlate well with the millimeter-fluxes during the time of the
EGRET observations.
% (In the First EGRET Catalog \citep{1eg} the following radio 
% catalog references are given for the source identifications:
% \citet{kuehr81, waltman91, wiren92, brown89}.)

Also, there are several AGNs with radio
spectra that are inverted up to the millimeter-domain at least during
their active state \citep{tornikoski00b, tornikoski01}, so the
low-frequency fluxes in general do not give an accurate estimate of
the millimeter-wavelength brightness of these sources.

In order to study the lower-confidence AGN identifications in 3EG we
used millimeter-data from our group's SEST observations (90 and
230 GHz), complemented with data from the literature, as described in
Section~\ref{sec:obs}.  For most of the sources we have relatively long data
sets which gives us a good estimate of their flux history and the
overall variability behavior, and for some sources we had data taken
(semi-)simultaneously with EGRET. We also studied the complete
radio continuum spectra of these sources.  Even most of the 5 GHz fluxes from 
\citet{tornikoski93}, Bramwell 1993 (private communication), 
\citet{reich98} and
\citet{reich00} are newer than the catalog data used by \citet{mattox01}.

We have made SEST observations for 12 of the 13 Southern Hemisphere
sources classified as lower-confidence identifications with AGNs in
3EG, and we discuss them here individually. The source 
\objectname[]{B1920$-$211}
(\objectname[]{J1923$-$2104}), 
given as a potential counterpart for \objectname[]{3EG\,J1921-2015}
in 3EG and considered ``a plausible EGRET identification'' in
\citet{mattox01} has not been observed by our group.

The radio spectra of these twelve sources are shown in
Figs.~\ref{fig3}--\ref{fig4}, and the flux density curves of the
sources for which we have time variability data are shown in
Figs.~\ref{fig5}--\ref{fig13}. In the following, the abbreviation BLO
stands for a BL Lacertae Object, LPQ for a Low Polarization Quasar
with a measured optical polarization $p<3\%$, HPQ for a Highly
Polarized Quasar with a measured optical polarization $p>3\%$, and QSO
for a quasar with no polarization information available.


\paragraph{B0130$-$171, a possible counterpart for 3EG\,J0130$-$1758}
($z$=1.020, QSO, Fig.~\ref{fig3} panel a.) 
A quasar for which there are only two 90 GHz data points taken more
than 14 years apart, and no 230 GHz data points. The difference
between the flux densities of these two 90 GHz points shows that the
source can be extremely variable.  With the more recent data point the
radio spectrum becomes inverted up to the millimeter-domain (with
$\alpha_{5,90;active}=+0.20$). We consider
\objectname[]{B0130$-$171} a likely counterpart for 
\objectname[]{3EG\,J0130$-$1758}.
% Joo.

\paragraph{B0506$-$612, a possible counterpart for 3EG\,J0512$-$6150}
($z$=1.093, LPQ, Fig.~\ref{fig3} panel b and Fig.\ref{fig5}.) 
The radio flux curves, even though sparsely sampled, show no
signs of any prominent activity and the flux levels remain relatively
low (ca. 1--1.5 Jy at 90 GHz, ca. 0.5 Jy at 230 GHz), also at the time
of the EGRET detections. The radio spectrum is relatively flat
($\alpha_{5,90;active}=-0.13$) but the spectral index remains almost
constant from the minima to the maxima. 
% Ehka.

\paragraph{B0521$-$365, a possible counterpart for 3EG\,J0530$-$3626}
($z$=0.061, BLO, Fig.~\ref{fig3} panel c and Fig.~\ref{fig6}.) 
This BL Lacertae Object has been monitored
with the SEST frequently before and during the Compton mission, and it
shows relatively high flux levels (over 5 Jy at 90 GHz) and complex
variability.  The EGRET detections with $(TS)^{1/2} \ge 2$ were made
close to the increased activity at the high radio frequencies.
The radio spectrum of this source is steep in the quiet
state: $\alpha < -0.50$ all the way from very low radio frequencies to
the millimeter-domain, and even in the active state the radio spectrum
does not flatten considerably ($\alpha_{5,90;active}=-0.21$), i.e.
remains steeper than for typical EGRET-detected blazars in spite
of the variability. 
% Ehka.

\paragraph{B0537$-$286, a possible counterpart for 3EG\,J0531$-$2940}
($z$=3.104, LPQ, Figs.~\ref{fig3} panel d and Fig.~\ref{fig7}.) 
A very high-redshift quasar for which there is only
little radio data. The radio spectrum is inverted up to 10 GHz in the
active state, but becomes steeper after that and remains very faint in
the millimeter-domain ($S_{230}< 0.2$ Jy, $S_{90}< 0.6$ Jy), also during the
EGRET pointings. The lack of activity at the high frequencies and the
steepness of the high-frequency part of the radio spectrum
($\alpha_{5,90}$ ranging from $-0.32$ to $-0.20$) suggest that this
source is not a likely counterpart for \objectname[]{3EG\,J0531$-$2940}. 
% Ei.

\paragraph{B0539$-$057, a possible counterpart for 3EG\,J0542$-$0655}
($z$=0.839, QSO, Fig.~\ref{fig3} panel e and Fig.~\ref{fig8}.) 
A quasar which has always been faint at 90 GHz, and below
the detection limit at 230 GHz (with an $4\sigma$ upper limit
$S_{230}\approx 250$ mJy). The high-frequency part of the
radio spectrum is steep, with $\alpha_{5,90}\approx -0.40$.
The low activity in the millimeter region and the steep radio spectrum
suggest that \objectname[]{B0539$-$057} is not the likely counterpart 
of \objectname[]{3EG\,J0542$-$0655}.
% Ei.

\paragraph{B0805$-$077, a possible counterpart for 3EG\,J0812$-$0646}
($z$=1.837, QSO, Fig.~\ref{fig3} panel f and Fig.~\ref{fig9}.) 
For this source there are only few EGRET data points.
The 90 GHz flux level has remained relatively constant ($S_{90}< 1.5$
Jy) from 1987 to 1994, also during the EGRET pointings, even though
moderate increase in the millimeter-fluxes can be seen at the time of 
the EGRET detections.
The 90 GHz flux, however, shows
a prominent increase ($S_{90}=2.5$ Jy) in the beginning of 1997, after
which we unfortunately have no further data.  The radio spectrum is
flat ($\alpha_{5,90}=-0.07$) up to 100 GHz in the quiet state, and
becomes extremely inverted ($\alpha_{5,90}=+0.30$) in the active
state. The flat to inverted high-frequency part of the radio spectrum
indicates considerable millimeter-wavelength activity in this source, and we
consider it a likely counterpart for \objectname[]{3EG\,J0812$-$0646}.
% Joo.

\paragraph{B1127$-$145, a possible counterpart for 3EG\,J1134$-$1530}
($z$=1.187, LPQ, Fig.~\ref{fig4} panel a and Fig.~\ref{fig10}.) 
The increase in the EGRET fluxes in the beginning of 1993 and 1995
coincide with increased activity in the millimeter-domain, but the
high-frequency part of the radio spectrum remains steep even during
the active state ($\alpha_{5,90;active}=-0.33$).  This source,
however, is bright and variable all the way from the low-frequency to
the millimeter-domain, with the maximum flux densities at 90 GHz and
230 GHz being 2.15 Jy and 1.30 Jy, respectively.  In spite of the
somewhat steep high-frequency part of the radio spectrum we consider
this source a possible EGRET counterpart.
% Joo.

\paragraph{B1313$-$333, a possible counterpart for 3EG\,J1314$-$3431}
($z$=1.210, QSO, Fig.~\ref{fig4} panel b and Fig.~\ref{fig11}.) 
This source is very variable at the high radio-frequencies.
The  EGRET detections with $(TS)^{1/2} \ge 2$ have been made at the increase of
millimeter-wavelength activity, from the end of 1991 to the first half of
1993. The radio spectrum of \objectname[]{B1313$-$333} is extremely
flat, ranging from $\alpha_{5,90}=-0.13$ in the quiet state to
$\alpha_{5,90}=+0.27$ in the active state. We consider
\objectname[]{B1313$-$333} a likely counterpart for
\objectname[]{3EG\,J1314$-$3431}.
% Joo.

\paragraph{B1504$-$166, a possible counterpart for 3EG\,J1504$-$1537}
($z$=0.876, HPQ, Fig.~\ref{fig4} panel c and Fig.~\ref{fig12}.)  
This source has only one significant EGRET detection.  An increase in
the radio flux levels coincides with the time of the EGRET detection,
even though no major radio outbursts are seen during that time. The
radio spectrum is flat between 1 and 30 GHz and then becomes steeper,
with $\alpha_{5,90}=-0.44$ in the quiet state to $\alpha_{5,90}=-0.26$
in the high state.
% !! Ghoshin pisteet spektriplotissa, joku pielessa? --> ei loydy
%    selitysta, antaa olla !!
% Ehka.

\paragraph{B1514$-$241 (AP Lib), a possible counterpart for 3EG\,J1517$-$2538}
($z$=0.049, BLO, Fig.~\ref{fig4} panel d and Fig.~\ref{fig13}.) 
A BL Lacertae object for which we have relatively long and densely
sampled millimeter flux curves that show increased activity at the
time of the increase in the EGRET fluxes (in 1992.2 and 1993.7--1994.8). 
The radio spectrum is very flat, ranging from
$\alpha_{5,90}=-0.16$ in the quiet state to $\alpha_{5,90}=+0.11$ in
the active state. We consider \objectname[]{B1514$-$241} a very likely
counterpart for \objectname[]{3EG\,J1517$-$2538}.  This object was
also the only source studied in our set of 12 sources that
\citet{mattox01} list as ``EGRET identifications with a high
probability of being correct''.
% Joo.

\paragraph{B1716$-$771, a possible counterpart for 3EG\,J1720$-$7820, and
J1808$-$501, a possible counterpart for 3EG\,J1806$-$5005}
For each of these two sources, both at an unknown redshift, we only
have one 90 GHz data point, taken after the end of EGRET
operation. For both of them the S/N of the 90 GHz observation was less
than 4, giving us only the upper limit of the source flux density
(with the $4 \sigma$ upper limit of $S_{90}<$ 370 mJy for
\objectname[]{B1716$-$771} and $S_{90}<$ 250 mJy for
\objectname[]{J1808$-$501}).  Since both of these sources were faint
at the time of the 90 GHz observation, they are neither very promising
candidates for strong millimeter-wave activity nor very good
candidates for the EGRET-detection counterparts.
% Ei & Ei.

\section{Millimeter-domain observations of promising candidates for 
new Southern EGRET identifications}
\label{sec:msm-cand}

As mentioned in Section~\ref{sec:lowconf},
\citet{mattox97}, hereafter MSM, have presented a method to make new
identifications for the unidentified sources listed in the 2nd EGRET
catalog \citep{thompson95}, hereafter 2EG, and the Supplement to the
2nd EGRET catalog \citep{thompson96}, hereafter 2EGS. MSM list 17
potential identifications for which they expect further investigation
to be most useful.

When we exclude all Northern Hemisphere sources and all the sources that
have been either left out from 3EG (after the improved analysis), or
identified in the 3EG (including the two sources identified using our
group's observations in \citet{bloom97}), this leaves us with two
sources: \objectname[]{J1249$-$8303} and \objectname[]{J1650$-$5044}, 
Table~\ref{tab:msm_promising}.
The table lists the observed AGN counterpart, the possible EGRET
catalog identification, the epoch of the 90 GHz observation, the flux
density at 90 GHz for the source that was detected at a significant
(signal-to-noise $\ge$4.0) level, and the spectral index between 5 and
90 GHz.

The first of the two ``promising candidates'' from MSM,
\objectname[]{J1249$-$8303}, was not detected at a significant level
at 90 GHz (the $4 \sigma$ upper limit of $S_{90}$ being 308 mJy).  The
second source, \objectname[]{J1650$-$5044}, has a millimeter flux and
spectrum which would make it a very promising candidate for the AGN
counterpart of the EGRET detection, but in 3EG Hartman et al. comment
that the corresponding 3EG source, \objectname[]{3EG\,J1638$-$5155},
lies $2^{\circ}$ away from the 2EG position, and that the possible
identification by MSM of \objectname[]{2EG\,J1648$-$5042} with this
AGN thus appears less convincing.

MSM list radio sources as potential source identifications for the 2EG
and the 2EGS with the probabilities of correct identification, based
on the radio source position and flux density.  The sources with the
highest probabilities have been identified with the 2EG/2EGS sources
either in the same paper (Table 3 of MSM, ``EGRET identifications with
a high probability of being correct'') or later in 3EG.  The
probability listed in MSM is, however, based on low frequency radio
fluxes of the Parkes-MIT-NRAO survey \citep{griffith93}, hereafter
PMN, obtained
in 1990 at 4.8 GHz.  We have composed a list of candidates for radio
identifications of EGRET sources from MSM Tables 1 and 2, excluding
all Northern Hemisphere sources, all sources that have been identified
in MSM, or in \citet{bloom97}, or in 3EG, and all that have the
identification probability $p < 0.001$. 

There were 20 sources left in our sample, and we managed to observe 19
of them, shown in Table~\ref{tab:msm_possible}.  The columns are as in
Table~\ref{tab:msm_promising}, except that the flux density column
shows ``N.D.'' (``no detection'') for sources with a signal-to-noise
(S/N) ratio $<$2.0, and the marginal detections, with $2.0< S/N <
4.0$, have an S/N entry included in the flux density column. All other
data points have S/N $\ge$ 4.0 and are considered detections at a
significant level. The flux limit for a S/N $\ge$ 4 detections under
these observing conditions was approximately 350 mJy.  The integration
times chosen for most of these sources were 14 $\times$ 25 s for each
beam in the dual beam switching mode, optimized for sources that would
be in the order of $S_{90} \ge 0.5$ Jy, i.e. ``brighter than
expected''.

Three of the sources (\objectname[]{J0726$-$4728},
\objectname[]{J1058$-$8003}, \objectname[]{J1703$-$6212}) with a
significant ($S/N>4$) detection at 90 GHz have a flat 5 to 90 GHz
spectrum, which make them good candidates for the counterparts of the
EGRET detections. \objectname[]{J1058$-$8003} and
\objectname[]{J1703$-$6212} have a 90 GHz flux density $S_{90}>1$ Jy
in this one-epoch data set, showing that these sources can be bright
in the millimeter-domain, and having a flat spectrum up to the 100 GHz
region they are probably variable sources with blazar-like
characteristics in the millimeter-domain.

In addition to the aforementioned objects, there are two sources
(\objectname[]{J1454$-$1925} and \objectname[]{J2005$-$2310}) with a
marginal 90 GHz detection and a relatively flat spectrum.  It is worth
making further millimeter-wave observations of these sources at some
other epochs to see whether their high radio-frequency variability
corresponds to that of other flat-spectrum sources and thus makes them
possible candidates for the EGRET identifications.


\section{Possible candidates for new Southern 3EG EGRET identifications}
\label{sec:newcand}

In order to try to identify AGN counterparts for 3EG sources with no
identifications in the catalog, we chose potential new counterpart
candidates according to the following criteria:

\begin{itemize}
\item
  Only Southern Hemisphere sources with $|b| > 10^{\circ}$ were included.
\item 
  Only sources with no entry in 2EG or 2EGS were included, i.e., these are new 
sources not earlier discussed by MSM, and also these were more likely to be in 
an active radio state during our observations in 2000 if compared to ``old''
EGRET detections from 2EG.
\item 
  We searched for the candidates within a $1^{\circ}$ radius of the 3EG 
  position, which is the size of the EGRET error box.
\item 
  In the first stage we selected candidates which are radio sources from 
  the PMN survey or the Parkes survey \citep{wright90}, hereafter PKS. 
  There were several candidates for each 3EG source.
\item
  From the candidates we excluded all of those that had a 5 GHz flux in
  the PMN survey less than 200 mJy. Even though
  the PMN fluxes are one-epoch fluxes obtained almost 10 years earlier of
  sources which are expected to be highly variable, it is reasonable to
  assume that very few if any of the extremely faint sources would be
  detected in the millimeter-domain with the current sensitivity of the
  SEST instruments.
\end{itemize}

We observed 19 of the EGRET source counterpart candidates at 90 GHz in
April 2000 and/or August 2000. They are shown in
Table~\ref{tab:omat_possible}.  Among these sources there are two
significant detections ($S/N\ge 4.0$) at 90 GHz. One of them,
\objectname[]{J1819$-$6345}, is a known compact steep-spectrum source,
and our 90 GHz data point confirms the steepness of its spectrum. It
is an unlikely AGN counterpart for an EGRET-detection.

\objectname[]{J1605$-$1139} has a flux density of 530 mJy at 90 GHz
and a 5 to 90 GHz spectral index $\alpha=+0.12$, making it a good
candidate for millimeter-range activity. This source is located within
$50.6'$ of the EGRET position of \objectname[]{3EG\,J1607$-$1101}.

There were three other flat-spectrum ($\alpha >-0.20$) sources
(\objectname[]{J0710$-$3850}, \objectname[]{J1254$-$4425} and 
\objectname[]{J2249$-$1251})
detected at a marginal level at 90 GHz. Since our study of this sample
was made using 1--2 observing epochs only, the millimeter-variability behavior
of these sources is currently unknown, but the flatness of their
spectra indicates that they could be active also at the higher
radio frequencies.

Because all the sources studied in this sample were observed {\it after}
the EGRET was operational, it is impossible to estimate what the
millimeter flux densities were {\it during} the EGRET detections,
and we can only conclude that the sources that were now detected
at a significant level can at least sometimes be bright at 90 GHz.

\section{Conclusions}
\label{sec:conclusions}

We have studied high radio-frequency (90 and 230 GHz) data of southern
AGNs that are possible counterparts for the sources listed in the
Third EGRET Catalog.

The high-confidence AGN identifications in 3EG are objects that are
bright and variable in the millimeter-domain, having a flat spectrum up to
$\nu \approx 100$ GHz, and in our search for EGRET source counterparts we
tried to find sources that have flat spectra and show millimeter-wavelength
activity, preferably close to the time of the EGRET detection.

We first studied the sample of Southern sources classified as
``possible AGN identifications'' in 3EG. We have millimeter-wavelength data
for 12 of them (out of 13), ranging from one-epoch observations to
more than 65 data points at 90 GHz from a period of more than 13 years.
The objects \objectname[]{B0130$-$171}, \objectname[]{B0805$-$077},
\objectname[]{B1313$-$333} and \objectname[]{B1514$-$241} were found
to have extremely flat spectra (inverted up to 100 GHz in the active
state) and to be bright and variable in the millimeter-domain (with
$S_{90,max}$ ranging from 1.76 Jy to 3.13 Jy), thus confirming the
identification. \objectname[]{B1127$-$145} has a somewhat steeper
spectrum but can still be the correct identification.  For some of the
aforementioned sources we have data that show that the gamma-detection
was made at the time of increased activity at high radio-frequencies.

The objects \objectname[]{B0537$-$286}, \objectname[]{B0539$-$057}, 
\objectname[]{B1716$-$771} and \objectname[]{J1808$-$5011}
have steeper spectra ($\alpha_{5,90}<-0.20$ even in the active state)
and all are faint at high radio-frequencies (with the largest
$S_{90,max}=$580 mJy) . We do not consider them
likely counterparts of the 3EG sources.

The remaining objects, \objectname[]{B0506$-$612},
\objectname[]{B0521$-$365} and \objectname[]{B1504$-$166} are all
relatively bright and active in the millimeter-domain and even though they
show blazar-like characteristics and could be the correct
identifications for the 3EG counterparts, they lack some features of
the aforementioned convincing cases.

We have also studied the millimeter flux densities and the 5 to 90 GHz
spectra of sources that were considered possible identifications for
the so far unidentified EGRET sources. We suggest that the following
four EGRET sources can possibly be identified with an AGN:
\objectname[]{2EG\,0720$-$4746} (id: \objectname[]{J0726$-$4728}), 
\objectname[]{2EGS\,1050$-$7650} (id:
\objectname[]{J1058$-$8003}), \objectname[]{2EGS\,1703$-$6302} (id: 
\objectname[]{J1703$-$6212}), and
\objectname[]{3EG\,J1607$-$1101} (id: \objectname[]{J1605$-$1139}). 
In addition to these, we found
that five other AGNs, though faint at 90 GHz, seem to have flat 5 to
90 GHz spectra, which indicates millimeter-activity that may be related to the
gamma-ray activity.

\acknowledgments
We acknowledge the support of the Academy of Finland for this work.
The SEST is operated jointly by ESO and the Swedish National Facility
for Radio Astronomy, Onsala Space Observatory at Chalmers University
of Technology.  This research has 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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\clearpage

\begin{figure}
\figurenum{1}    % Spektri-indeksihistogrammi, A
\epsscale{0.9}
\plotone{f1.eps}
\caption{Spectral index distribution of the sources in Table~\ref{tab:highconf}
(3EG high-confidence identifications with AGNs). 
\label{fig1}}
\end{figure}

\begin{figure}
\figurenum{2}  % Spektri-indeksihistogrammi, a
\epsscale{0.9}
\plotone{f2.eps}
\caption{Spectral index distribution of the sources in Table~\ref{tab:lowconf}
(3EG lower-confidence identifications with Southern AGNs). 
\label{fig2}}
\end{figure}
 
\begin{figure}
\figurenum{3}   % Spektriplotti, 1a-f
\epsscale{0.9}
\plotone{f3.eps}
\caption
{Radio spectra of AGNs that are lower-confidence identifications
of the EGRET sources in 3EG: (a) B0130$-$171, (b) B0506$-$612, (c) 
B0521$-$365, (d) B0537$-$286, (e) B0539$-$057, (f) B0805$-$077.
\label{fig3}}
\end{figure}

\begin{figure}
\figurenum{4}  % Spektriplotti, 2a-f
\epsscale{0.9}
\plotone{f4.eps}
\caption
{Radio spectra of AGNs that are lower-confidence identifications
of the EGRET sources in 3EG: (a) B1127$-$145, (b) B1313$-$333, 
(c) B1504$-$166, (d) B1514$-$241.
\label{fig4}}
\end{figure}
 
\begin{figure}
\figurenum{5}  % Fluksiplotti, 0506$-$612
\epsscale{0.9}
\plotone{f5.eps}
\caption
{Flux density curves for B0506$-$612. The top panel shows the 
EGRET observations with $(TS)^{1/2} \ge 2$ with error bars, filled circles,
and the EGRET upper limits with $(TS)^{1/2} < 2$, open circles.
The 230 GHz flux curve is shown in the middle, and the 90 GHz
flux curve at the bottom.
\label{fig5}}
\end{figure}
 
\begin{figure}
\figurenum{6}  % Fluksiplotti, 0521$-$365
\epsscale{0.9}
\plotone{f6.eps}
\caption
{As Fig.~\ref{fig5} but for the source B0521$-$365.
\label{fig6}}
\end{figure}
 
\begin{figure}
\figurenum{7}  % Fluksiplotti, 0537$-$286
\epsscale{0.9}
\plotone{f7.eps}
\caption
{As Fig.~\ref{fig5} but for the source B0537$-$286.
\label{fig7}}
\end{figure}
 
\begin{figure}
\figurenum{8}  % Fluksiplotti, 0539$-$057
\epsscale{0.9}
\plotone{f8.eps}
\caption
{As Fig.~\ref{fig5} but for the source B0539$-$057, and
only the EGRET data and 90 GHz radio data were available.
\label{fig8}}
\end{figure}
 
\begin{figure}
\figurenum{9}  % Fluksiplotti, 0805$-$077
\epsscale{0.9}
\plotone{f9.eps}
\caption
{As Fig.~\ref{fig5} but for the source B0805$-$077
\label{fig9}}
\end{figure}
 
\begin{figure}
\figurenum{10}  % Fluksiplotti, 1127$-$145
\epsscale{0.9}
\plotone{f10.eps}
\caption
{As Fig.~\ref{fig5} but for the source B1127$-$145.
\label{fig10}}
\end{figure}

\begin{figure}
\figurenum{11}  % Fluksiplotti, 1313$-$333
\epsscale{0.9}
\plotone{f11.eps}
\caption
{As Fig.~\ref{fig5} but for the source B1313$-$333.
\label{fig11}}
\end{figure}
 
\begin{figure}
\figurenum{12}  % Fluksiplotti, 1504$-$166
\epsscale{0.9}
\plotone{f12.eps}
\caption
{As Fig.~\ref{fig5} but for the source B1504$-$166.
\label{fig12}}
\end{figure}
 
\begin{figure}
\figurenum{13}  % Fluksiplotti, 1514$-$241
\epsscale{0.9}
\plotone{f13.eps}
\caption
{As Fig.~\ref{fig5} but for the source B1514$-$241.
\label{fig13}}
\end{figure} 
 
% tables:
\input{tab1}        % tab_highconf
\input{tab2}        % tab_lowconf
\input{tab3}        % tab_msm_promising
\input{tab4}        % tab_msm_possible
\input{tab5}        % tab_omat_possible

\end{document}
