AGN Science

We have complemented our study of the complete sample of Southern BL Lacertae objects and quasars originally presented in Tornikoski et al. 1993, AJ 105, 1680. The new SEST data set includes 230 GHz data for all of these sources and numerous additional 90 GHz measurements, which enabled us to study the high radio-frequency spectra and variability of these sources in detail.

Many of the sources have flat spectra up to 90 GHz, but even at 230 GHz they remain bright enough to be detected at

mJy flux levels, many of them considerably brighter than that. When we construct an average spectrum using the spectral shape from the sample sources, we find that sources as faint as 350 mJy at 2.3 GHz should be detected at 230 GHz with the current SEST setup, indicating that a large number of sources fainter at the lower frequencies that get excluded from the ``complete'' samples should be easily detectable in the millimeter domain. Many of these sources probably exhibit interesting high radio frequency variability behavior and studying them more closely could offer us new information about the millimeter tail of the radio spectrum.

We have analyzed the millimeter-domain variability of our sample sources, finding that with random data sampling, only 1/5 of the observations at 90 GHz and 1/10 at 230 GHz detect BLOs and HPQs in the flaring state. The variability behaviour should be taken into account when estimating the source behavior based on only few data points, likely to have been observed during the intermediate or quiescent state.

The results of this study were published in Tornikoski et al. 2000, AJ 1739, 2278.

When analysing our complete sample of southern sources, we identified two new candidates for Gigahertz Peaked Spectrum (GPS) sources. These two sources are PKS 1936

155 and PKS 2008

159, both peaking at unusually high frequencies, ca. 10

20 GHz in the observer's frame (See Figure 18).

The radio continuum spectra of these two sources were published in Tornikoski et al. 2000, AJ 1739, 2278. In this paper we also suggest that probably 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.

**Figure 18:** A new gigahertz-peaked spectrum (GPS) source PKS1936155 was identified by our group (Tornikoski et al. 2000, Astronomical Journal 120, 2278). The data points presented in the figure were mainly observed by our group and our collaborators over a time span of 8 years. The red solid line shows the shape of the canonical GPS spectrum given by de Vries et al. (Astronomy and Astrophysics 321, 105, 1997). PKS1936155 has a spectral shape of a typical GPS source, but the peak frequency is exceptionally high, ca. 20 GHz, whereas ``ordinary'' GPS sources have peak frequencies just slightly above 1 GHz. The long term monitoring of this source shows that it is a highly variable source across the whole radio spectrum. Even though GPS sources have been considered to be among the least variable extragalactic sources, we suggest that this kind of variability is typical for the quasar-type high peaking GPS sources.
$\begin{figure}\begin{center} \epsfig{file=1936_bw2.eps, width=9cm} \end{center}\end{figure}$

We continued our search for new GPS source candidates by carefully examining our full SEST data base, $\sim$ 100 sources with at least one data point at 90 GHz and/or 230 GHz with $S/N\ge4$ , almost all of which are quasars or BL Lacertae objects. We 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 presented 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.

The spectra of the new GPS source candidates and other sources with inverted spectra will be published in Tornikoski et al., AJ, in press.

Among the sources in our complete sample of Southern sources (see above) there were two bona fide GPS sources that exhibited radio variability behaviour similar to the average sources in our sample, and also similar to our two newly identified GPS sources. We wanted to study the variability of also other known GPS sources, and we found the variability of the known quasar-type GPS sources observed by our group to be 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 at least for these quasar-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 sources are observed only few times, it is easy to misidentify a variable flat-spectrum radio source as a GPS source if the source has been in an active state during the observation.

The results about the variability of bona fide GPS sources will be published in AJ together with the new GPS identifications.

During the summer 2000 a research on BL Lacertae objects was conducted. The aim of the research was to compile ''complete'' samples of different classes of BL Lacs by collected data from various databases. Four samples were made representing 1) all known BL Lacs, 2) the radio-selected BL Lacs (RBL), 3) the X-ray-selected BL Lacs (XBL) and 4) the Intermediate BL Lacs (IBL). For 75 radioloud BL Lacs radio data were found from the Metsähovi summary data file. For these objects logarithmic frequency vs. flux density plots were made. In addition to the power-plots linear variation-plots were made at different frequencies. The radio-selected BL Lacs were analysed in greater detail. Spectral index plots, variability plots and redshift plots were made and the properties of some of the interesting objects were collected from the literature. The results were published in the Bachelor's thesis of P. Johansson (Helsinki University, 2000)

The long term goal of the research is to understand how the new classes of BL Lacs behave at high radio frequencies, especially at the Planck satellite's frequency range. The various samples of the BL Lacs will be observed using the Metsähovi and SEST telescopes from 2001 onwards.

There have been several studies of ``complete'' samples of radio sources, each selected using different criteria, leading to the inclusion of different objects. When selecting good candidates for high radio-frequency observations, it is generally assumed that the radio spectra peak at low frequencies and get very steep in the millimeter range. Thus, only sources exceeding a certain flux limit (e.g., S

Jy at 5 GHz) and showing a flat spectrum at the low frequencies often end up being monitored in the millimeter domain. Also, the selection is often based on only one-epoch observations at the low frequency range. Thus there is a risk that a large number of interesting sources, especially the ones that are relatively faint but show strong variability in the millimeter domain, are excluded from the high radio-frequency studies.

We have studied at SEST a sample of sources fainter in the low-frequency domain to see whether the radio to millimeter to submillimeter spectral indices of the faint flat-spectrum sources are within the range found for the brighter flat-spectrum sources. Our initial results indicated that the fainter sources tend to have steeper than expected high-frequency spectra. The results will be published in 2001.

In 2000 we had two sessions of SEST observations during which we made 90 GHz observations of a sample of AGNs that we had chosen for possible candidates of some of the unidentified EGRET sources. These observations will be reduced and the results published in 2001.