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 Group of Active Galactic Nuclei investigation

Simultaneous spectra and radio properties of the BL Lacertae objects

BL Lacertae (BL Lac) objects are a rare subclass of active galactic nuclei (AGN) and they make up about 1% of all known AGNs (Plotkin et al. 2008). Electromagnetic emission in this class is over the entire energy spectrum, from the radio band to the most energetic -rays (Urry & Padovani 1995). They are characterized by nearly featureless optical spectra with no or very weak emission lines, strong and rapid variations in intensity and polarizations (Strittmatter et al. 1972; Kollgaard 1994; Urry & Padovani 1995; Perlman et al. 1998). Radio loudness is to be another of their fundamental properties (Stocke et al. 1990). It is considered that in the BL Lacs the relativistic jet is closely aligned to the line of sight ( < 20o) and relativistic effects play important role in the observed properties of the BL Lacs (Blandford & Rees 1978).

Figure 1. A schematic diagram of the current paradigm for radio-loud AGNs (Urry & Padovani 1995).

Due to the extremely strong magnetic fields present in the jets, their spectral energy distributions (SEDs) tend to be dominated by two non-termal radiation components (Sambruna et al. 1996): synchrotron and inverse Compton scattering (at the lower and higher frequencies, respectively).
Usually BLOs samples are based on the cross-correlating existing radio and X-ray survey catalogues (Perlman et al. 1998; Turriziani et al. 2007; Plotkin et al. 2008). Currently the most extensive list of the BL Lacertae objects is presented in the Roma-BZCAT catalogue by Massaro et al. (2009). This catalogue is based on multifrequency surveys and detailed checkout of the literature and contains 1180 BL Lacertae objects and candidates. But only a relative small number objects have been observed intensively at many frequencies simultaneously. The spectral coverage of many of them is poor, both in the time and in the frequency. In general, BL Lacs which have been discovered in either radio or X-ray band, that base their classification as radioselected (RBL) and X-ray-selected (XBL) BL Lacs.

In the BL Lacertae Object source list we have included approximately the same number of sources from all BL Lac subclasses: (HBLs, IBLs and LBLs). The difference between these objects is the peak frequency of the synchrotron component of their spectral energy distribution (HBLs: high-energy peak, LBLs: low-energy peak, IBLs: intermediate).
The class division has been made using parabolic fits to the synchrotron components as described in Nieppola et al. (2006). The original source list in Nieppola et al. 2006 is the largest BL Lac sample to date, which has been classified directly based on the SEDs instead of secondary properties such as spectral indices. It was founded the strong correlation between the energy of the radiating electrons and the total energy density of the emitting region peak ~ (Ur +UB)-0.6. The result is that the subclass LBLs has an average higher luminosity than other.
HBLs are, on the whole, very faint in the radio domain, and many of them have very few radio data points (at 37 GHz, only approximately 15% of the class with S / N > 4). Any information on their spectral behaviour and flux levels would be of great use, including knowledge of their radio-faintness or even possible radio-quietness. This is of course also true for the majority of LBLs and IBLs, most of which have little radio data

The BLO sample includes 108 objects selected from the Metsahovi BL Lac sample (Nieppola et al. 2006). It is one of the largest BLO sample ever studied and consists of 398 objects, where 382 are from the Veron-Cetty & Veron BLO catalogue (Veron-Cetty & Veron 2000) and 17 are from the literature. Almost all objects of the sample were classified as LBL, IBL and HBL based on their synchrotron peak frequencies, peak :
log peak 14.5 for LBLs, 14.5 < log peak 16.5 for IBLs and log peak > 16.5 for HBLs. The synchrotron peak frequencies were calculated from parabolic fit to their SEDs (Nieppola et al. 2006).
The observations were carried out with the RATAN-600 radio telescope during 2006-2011 at six frequencies: 1.1, 2.3, 4.8, 7.7, 11.2 and 21.7 GHz. The flux densities of 108 sources for several epochs with available RATAN-600 observations are given in Table 1.
  • Column 1 - source name [hhmmss+ddmmss] from the NVSS catalog;
  • Col. 2 - observation epochs at RATAN-600 [];
  • Cols. 3-14 - flux densities and their uncertainties for the corresponding frequencies, including the uncertainty in the source's antenna temperature and the calibration curve.

Averaged instantaneous spectra at several epochs are presented. Almost all sources have complete data at the frequencies 4.8 and 7.7 GHz. Absence of data for some sources at some frequencies is a result of data exclusion because of the following reasons: partial resolution of a source at some frequencies, a source is too weak to be measured reliably, a strong influence of man-made interferences (usually at 31 and 13 cm), strong interferences from the geostationary satellites at 11.2 GHz (in the declinations between - 10o and 0o. Nevertheless, in some cases the data was not excluded in spite of the increase in errors. The values of the standard error of fluxes is: 5% - 10% for 11.2, 7.7, and 4.8 GHz, 7% - 20% for 2.3, 1.0, and 21.7 GHz.