Multifrequency studies of gigahertz-peaked spectrum sources and candidates

Gigahertz-peaked spectrum (GPS) sources are compact radio sources located in centres of distant active galaxies. The shape of the radio continuum spectrum of GPS sources is convex, the flux density increases towards high frequencies and decreases above a turnover at gigahertz-frequencies. Whereas the majority of extragalactic radio sources extend well outside of their host galaxies, these compact sources reside only in the central regions of their host. For this reason studying GPS sources can provide us with information on the structure and the properties of active galactic nuclei. In this thesis, the radio spectra and the total flux density variability of GPS sources are studied. We have collected an extensive database of radio observations of GPS sources and candidates from the monitoring programmes of Metsähovi Radio Observatory and University of Michigan Radio Astronomy Observatory, and from the literature. We have also made new observations of the sources. In the literature, the GPS classification is often done using few non-simultaneous data points presuming that the sources are not variable. Our results show that this approach has produced samples that are heavily contaminated by misclassified sources. Among the quasar-type GPS sources classified in the literature, there is only a small fraction of genuine GPS sources as highly variable sources with temporary GPS features in their radio spectrum have been misclassified as GPS sources. The fraction of genuine GPS galaxies was found to be larger but the contamination was significant also in galaxy-type samples. In addition, the genuine GPS sources were found to be variable contrary to earlier conception. Cluster analyses of GPS sources presented in this thesis also support the view of heterogeneous GPS samples. The blazar-type sources with temporarily gigahertz-peaked spectra clearly stood out as their own cluster, and the sources with confirmed GPS-type spectra formed several different clusters. This result supports the view that there are different types of genuine GPS sources

37 GHz observations of a large sample of BL Lacertae objects

We present 37 GHz data obtained at Metsahovi Radio Observatory in 2001 December - 2005 April for a large sample of BL Lacertae objects. We also report the mean variability indices and radio spectral indices in frequency intervals 5 - 37 GHz and 37 - 90 GHz. Approximately 34 % of the sample was detected at 37 GHz, 136 BL Lacertae objects in all. A large majority of the detected sources were low-energy BL Lacs (LBLs). The variability index values of the sample were diverse, the mean fractional variability of the sample being \Delta S_2 = 0.31. The spectral indices also varied widely, but the average radio spectrum of the sample sources is flat. Our observations show that many of the high-energy BL Lacs (HBL), which are usually considered radio-quiet, can at times be detected at 37 GHz.Comment: 12 pages, 5 figures + 5 tables. Published in Astronomical Journa

37 GHz observations of a large sample of BL Lacertae objects

ABSTRACT We present 37 GHz data obtained at Metsähovi Radio Observatory from 2001 December to 2005 April for a large sample of BL Lacertae objects. We also report the mean variability indices and radio spectral indices in frequency intervals 5Y37 and 37Y90 GHz. Approximately 34% of the sample was detected at 37 GHz, 136 BL Lacertae objects in all. A large majority of the detected sources were low-energy BL Lac objects. The variability index values of the sample were diverse, the mean fractional variability of the sample being ÁS 2 ¼ 0:31. The spectral indices also varied widely, but the average radio spectrum of the sample sources is flat. Our observations show that many of the highenergy BL Lac objects, which are usually considered radio-quiet, can at times be detected at 37 GHz