Extreme GPS and other strongly inverted spectrum radio sources



GPS (Gigahertz Peaked Spectrum) sources are bright and compact radio sources with exceptionally high frequency peaked spectra (in other words, they have "inverted" spectra). The current working theory describes them as young active galaxies in the early stages of development, before expansion to a larger size. This explains the exceptionally high-peaked spectra -the spectral peak of an expanding radio source moves towards lower frequencies.

GPS source PKS 1936-155, N.B. the spectral
                      peak at gigahertz frequencies Planck satellite is very well suited for observations of GPS sources. Its receivers at nine different frequencies will produce a spectrum for each source -GPS sources will be easily recognized. However, because their spectra peakes at such high frequencies, they may hamper the CMB measurements. This is why it is important to study also GPS sources before the launch.

Our quasar research group in Metsähovi and Tuorla has found out that, contrary to usual view, quasar-like GPS sources seem to be strongly variable. It is possible that many sources that have so far been classified as GPS sources are actually normal active galaxies, whose spectra have been put together from observations taken at different times (for example, the peak of the spectra has been measured during a strong radio flare, while all other measurements have been made during a quiescent period). The variability data, at several timescales, provided by the Planck satellite will shed light on the problem. It is going to be exciting to see how many GPS sources there really are, to measure their polarization, and to study how they are connected to the other types of active galaxies.

Other kinds of sources can also have inverted spectra, for example, nearby usual galaxies that harbour supermassive but quiet black holes in their centres. They are believed to be numerous but rather faint. Also dusty active galaxies and radio afterglows of gamma-ray bursts can produce an inverted spectrum.

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