Observations and analysis of active galactic nuclei for Planck satellite |
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We have started new observing and data analysis programmes for the Planck
project. Our source samples include objects that have never been observed
at these frequencies before, and the amount of data analysed is unprecedented.
At the moment our source list includes approximately 700 sources.
Not only are the observations for the Pre-launch Catalog important in
themselves, but we need to know the behaviour and physics at work in these
sources as well. This knowledge is required both for the success of the CMB
mapping as well as for the non-CMB science. The idea was to study new source populations and to get an estimate of how they affect the Planck mission. In the past most of these sources have been excluded from high frequency studies because they are believed to be weak in the radio domain. However, our new observations show that many (possibly hundreds) of these sources are so bright in their active state that the Planck satellite can easily detect them. BL Lac Objects (BLOs)We started to observe a large sample of BLOs in 2001 with the Metsähovi telescope at 37 GHz. Most of the X-ray selected and intermediate BLOs have never been observed at high radio frequencies before, and we wanted to see whether this is justified or not. By autumn 2003 we had covered 98 % of the sample (about 400 sources). Currently most of them have been observed several times to look for variability. Of all these sources more than one third were detected, and about one third of the X-ray and intermediate BLOs, too, that have previously been thought to be too weak at radio frequencies! The detection rate is higher than expected, and some high-peaking BLOs seem to be well detectable also at lower frequencies, at least in their active state. This automatically means that these sources can also be detected by the Planck satellite!
References: Nieppola et al. "Spectral energy distributions of a large sample of BL Lacertae objects", Astronomy & Astrophysics 445, 441 (2006) Nieppola et al. "37 GHz observations of a large sample of BL Lacertae objects", Astronomy & Astroophysics, submitted (2006) Gigahertz-peaked spectrum (GPS) sourcesWe have been observing new samples of GPS sources with the Metsähovi telescope at 37 GHz since November 2001, and with the SEST at 90 GHz. The purpose is to search for new high-peaked sources, to study the variability of the bona fide GPS sources and the models used for describing them, and to study the impact of our findings on the Planck mission. Our group has identified several new extreme peaking GPS sources. It seems that the number of the sources with inverted spectra, and thus being bright in the mm wavelength domain, (previously excluded from high frequency studies) is larger that earlier assumed. We have also shown that many sources currently identified as bona fide GPS sources and candidates in the literature actually are ordinary flat spectrum sources with high variability that have inverted spectra only during flares.Our current working hypothesis is that the number of genuine GPS sources is smaller than the estimates given in the literature. However, the number of sources that sometimes, i.e. in their active state, peak at high radio frequencies seems to be higher than earlier assumed. This result is also of great importance for the Planck mission, because these sources can at times be extremely bright at the Planck frequency range. This emphasizes the importance of the prediction of source activity states during the mission, as well as the role of the radio flare modelling. References: Tornikoski et al. "Radio Spectra and Variability of Gigahertz-Peaked Spectrum Radio Sources and Candidates", Astronomical Journal 121, 1306 (2001) Torniainen et al. "Long term variability of gigahertz-peaked spectrum sources and candidates", Astronomy & Astrophysics 435, 839 (2005) Variability analysisOne of the tasks in both defining the parameters for our Quick Detection System as well as in planning multifrequency active galactic nuclei observing campaigns during the Planck mission is to study the long term millimetre variability of a large sample of sources, and so better understand their variability and variability timescale behaviour.
As a special interest for the Planck project, we are also looking at the possibility of predicting activity in a source based either on statistics or observed flux behaviour of the source. References: Valtaoja et al. "Total flux density variations in extragalactic radio sources. I. Decomposition of variations into exponential flares", Astrophysical Journal Supplement 120, 95 (1999) Lähteenmäki et al. "Total flux density variations in extragalactic radio sources. II. Determining the limiting brightness temperature for synchrotron sources", Astrophysical Journal 511, 112 (1999) Lähteenmäki & Valtaoja "Total flux density variations in extragalactic radio sources. III. Doppler boosting factors, Lorentz factors and viewing angles for active galactic nuclei", Astrophysical Journal 521, 493 (1999) WMAP unidentified and multiple identification sourcesIn cooperation with the Extragalactic Working Group we have observed the unidentified and multiple identification WMAP point sources immediately after the release of the WMAP catalog. We were able to identify several of them. The WMAP data are also an interesting guideline in the planning of the Planck mission. |
