High Frequency Peakers: Young radio sources or flaring blazars?^*

¹ SISSA/ISAS, via Beirut 4, 34014 Trieste, Italy e-mail: tinti@sissa.it
² Dipartimento di Astronomia, via Ranzani 1, 40127 Bologna, Italy
³ Istituto di Radioastronomia – CNR, via Gobetti 101, 40129 Bologna, Italy
⁴ INAF, Osservatorio Astronomico, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
⁵ Istituto di Radioastronomia – CNR, CP 169, 96017 Noto (SR), Italy

Received: 8 July 2004
Accepted: 26 October 2004

Abstract

We present new, simultaneous, multifrequency observations of 45 out of the 55 candidate High Frequency Peakers (HFP) selected by Dallacasa et al. ([CITE]), carried out 3 to 4 years after a first set of observations. Our sub-sample consists of 10 galaxies, 28 stellar objects (“quasars”) and 7 unidentified sources. Both sets of observations are sensitive enough to allow the detection of variability at the 10% level or lower. While galaxies do not show significant variability, most quasars do. Seven of them no longer show the convex spectrum which is the defining property of Gigahertz Peaked Spectrum (GPS)/HFP sources and are interpreted as blazars caught by Dallacasa et al. ([CITE]) during a flare, when a highly self-absorbed component dominated the emission. In general, the variability properties (amplitude, timescales, correlation between peak luminosity and peak frequency of the flaring component) of the quasar sub-sample resemble those of blazars. We thus conclude that most HFP candidates identified with quasars may well be flaring blazars.