Inverted-spectrum sources
Inverted-spectrum sources are extragalactic radio sources with a special shape of the radio continuum spectrum: whereas most of the active galactic nuclei have a simple flat or steep spectrum, the spectrum of inverted-spectrum sources is convex at gigahertz-frequencies (Fig. 1). The convex shape of the spectrum can be temporary if a source is a flaring flat-spectrum radio source, or it can be a constant feature of the spectrum, as in genuine gigahertz-peaked spectrum (GPS) sources and high frequency peakers (HFP).
Figure 1. Examples of AGNs with a) a steep spectrum, b) a flat spectrum, c) a genuine gigahertz-peaked spectrum, and d) a flaring blazar with temporarily inverted spectrum.
GPS sources and HFPs are currently considered to represent newborn radio sources, some cases have been estimated to be ignited only some hundreds of years ago (in the rest frame). Therefore, studying them will help us understand the origin and the evolution of AGNs. In Metsähovi, GPS sources and HFPs are monitored at 37 GHz. Studying the data archives and making new observations have revealed that a substantial proportion of GPS sources and HFPs presented in the literature are actually flaring blazars, i.e. the samples are heavily contaminated by other than young AGN.
For details on the GPS studies in Metsähovi, see 2007A&A...469..451T, 2005A&A...435..839T, 2001AJ....121.1306T.
We have analysed various properties of a sample of 206 GPS sources and HFPs with self-organized neural maps, and indeed, the misclassified flaring blazars stand out clearly from the genuine GPS sources. However, the genuine sources are also divided into different groups, which probably represent different types of sources. This clustering will be studied in more detail in a forthcoming article.