Long-term radio behaviour of GPS sources and candidates

This paper is a summary of the work that our group has done (and recently published in several papers) on long-term radio variability of GPS sources. We have studied the long-term (up to 30 years) variability of GPS sources and candidates, with emphasis on the high-frequency radio domain. Our data sets show that only a relatively small number of these sources retain their convex spectra when they are monitored densely and for long periods of time. The current GPS samples are especially contaminated by small, beamed blazar-type sources. Also the remaining population with consistently convex GPS-type spectra seems to be heterogeneous, falling into several subpopulations when their observed properties are used for clustering them through a self-organizing map. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim