Multifrequency observations of a sample of very low frequency peaked BL Lacertae objects

摘要

Context. BLLacertae objects with anextremely low value of the peak frequency (Hz)of the synchrotron component (VLBLobjects) are poorly known. Aims. We selected a small sample of these candidateobjects on the basis of their radio/optical flux ratio to evaluatetheir variability, define the low energy part of their SED and obtain areliable estimate of theirthrough a fit with a log-paraboliclaw. Methods. They were the object of a ground-basedobservational campaign in the infrared and optical bands; data fromseveral pointings by the Swiftsatellitein the UV and X-ray band were also obtained for half the sample. Wecomplemented our data with a large number of literature data. Results. These sources showed a marked fluxvariability, asexpected for BLLacertae objects ofthe low energy peaked BL Lac objects (LBL) and intermediate BL Lacobjectstype; all of them have avalue around 1013Hz,but this is low enough only for afew sources to consider themgenuine VLBLobjects. We report the overall properties ofindividual sources and discuss some aspects of this class of rareobjects, which could correspond to the transition between flat spectrumradio quasars andLBLs. Key words: galaxies: active - BL Lacertaeobjects: general - radiation mechanisms: non-thermal