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Register a new userSpectral energy distributions of a large sample of BL Lacertae objects
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We have collected a large amount of multifrequency data for the objects in the Metsahovi Radio Observatory BL Lacertae sample and computed their spectral energy distributions (SED). This is the first time the SEDs of BL Lacs have been studied with a sample of over 300 objects. The synchrotron components of the SEDs were fitted with a parabolic function to determine the synchrotron peak frequency. We checked the dependence between luminosities at several frequency bands and synchrotron peak frequency to test the blazar sequence scenario, which states that the source luminosity depends on the location of the synchrotron peak. We also calculated broad band spectral indices and plotted them against each other and the peak frequency. The range of peak frequencies in our study was considerably extended compared to previous studies. There were 22 objects for which log u_{peak}>19. The data shows that at 5 GHz, 37 GHz and 5500 A there is negative correlation between luminosity and nu_{peak}. There is no significant correlation between source luminosity at synchrotron peak and peak frequency. Several low radio luminosity-low energy peaked BL Lacs were found. The negative correlation between broad band spectral indices and nu_{peak} is also significant, although there is substantial scatter. Therefore we find that neither alpha_{rx} nor alpha_{ro} can be used to determine the synchrotron peak of BL Lacs. On the grounds of our results we conclude that the blazar sequence scenario is not valid. In all our results the BL Lac population is continuous with no hint of the bimodality of the first BL Lac samples.
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We present 37 GHz data obtained at Metsahovi Radio Observatory in 2001 December - 2005 April for a large sample of BL Lacertae objects. We also report the mean variability indices and radio spectral indices in frequency intervals 5 - 37 GHz and 37 - 90 GHz. Approximately 34 % of the sample was detected at 37 GHz, 136 BL Lacertae objects in all. A large majority of the detected sources were low-energy BL Lacs (LBLs). The variability index values of the sample were diverse, the mean fractional variability of the sample being Delta S_2 = 0.31. The spectral indices also varied widely, but the average radio spectrum of the sample sources is flat. Our observations show that many of the high-energy BL Lacs (HBL), which are usually considered radio-quiet, can at times be detected at 37 GHz.
BL Lac objects are active nuclei, hosted in massive elliptical galaxies, the emission of which is dominated by a relativistic jet closely aligned with the line of sight. This implies the existence of a parent population of sources with a misaligned jet, that have been identified with low-power radiogalaxies. The spectrum of BL Lacs, dominated by non-thermal emission over the whole electromagnetic range, together with bright compact radio cores, high luminosities, rapid and large amplitude flux variability at all frequencies and strong polarization make these sources an optimal laboratory for high energy astrophysics. A most distinctive characteristic of the class is the weakness or absence of spectral lines, that historically hindered the identification of their nature and ever thereafter proved to be a hurdle in the determination of their distance. In this paper we review the main observational facts that contribute to the present basic interpretation of this class of active galaxies. We overview the history of the BL Lac objects research field and their population as it emerged from multi-wavelength surveys. The properties of the flux variability and polarization, compared with those at radio, X-ray and gamma-ray frequencies, are summarized together with the present knowledge of the host galaxies, their environments, and central black hole masses. We focus this review on the optical observations, that played a crucial role in the early phase of BL Lacs studies, and, in spite of extensive radio, X-ray, and recently gamma-ray observations, could represent the future major contribution to the unveiling of the origin of these sources. In particular they could provide a firm conclusion on the long debated issue of the cosmic evolution of this class of active galactic nuclei and on the connection between formation of supermassive black holes and relativistic jets.
We review the main results from several radio, X-ray and multi-frequency surveys on the topic of cosmological evolution of BL Lacertae objects. Updated findings on BL Lac evolution following the recent identification of many sources in the ``Sedentary Multi-Frequency survey are also discussed. By means of extensive Monte Carlo simulations we test some possible explanations for the peculiar cosmological evolution of BL Lacs. We find that a dependence of the relativistic Doppler factor on radio luminosity (as expected within the beaming scenario) may induce low values of V/V_max and that both edge effects at the low luminosity end of the BL Lacs radio luminosity function, and incompleteness at faint optical magnitudes may be the cause of the low V/V_max found for extreme HBL sources in X-ray selected samples.
Observations of candidate BL Lacertae objects spectroscopically selected from the Sloan Digital Sky Survey (SDSS) reveal a large fraction with high polarization (P > 3%). This result confirms that synchrotron radiation makes an important contribution to the observed optical continuum for most objects in the sample. The SDSS sample can be divided into separate categories, with objects of undetermined redshift generally having the highest optical polarization. Polarization as high as 23% and the lack of spectral features suggests that the synchrotron continuum completely dominates the spectra of these sources. The mean polarization levels observed for objects having measured redshifts is much lower, with the maximum observed polarization for this group being ~10%. The lower polarizations of these objects are reminiscent of the less spectacular polarization levels shown by BL Lac objects discovered in X-ray surveys. We find no SDSS BL Lac candidates at z > 1 with P > 3%, calling their classification as BL Lac objects into question. In addition, the existence of radio-quiet BL Lac objects is not verified since none of 10 potentially radio-weak BL Lac candidates observed are highly polarized. Regardless of whether the high-redshift and radio-weak objects are included in this optical sample, the overall levels of polarization observed are intermediate between those seen for X-ray and radio-selected BL Lac objects.
Radio-bright BL Lacertae objects (BLOs) are typically variable and exhibit prominent flaring. We use a sample of 24 BLOs to get a clear idea of their flaring behavior and to find possible commonalities in their variability patterns. Our goal was to compare the results given by computational time scales and the observed variability parameters determined directly from the flux curves. Also, we wanted to find out if the BLO flares adhere to the generalized shock model. We use long-term monitoring data from 4.8, 8, 14.5, 22, 37, 90 and 230 GHz. The structure function, discrete correlation function and Lomb-Scargle periodogram time scales, calculated in a previous study, are analyzed in more detail. We determine flare durations, rise and decay times, absolute and relative peak fluxes from the monitoring data. We find that BLOs demonstrate a wide range of variability behavior. BLOs include sources with fast and strong variability, such as OJ 287, PKS 1749+096 and BL Lac, but also sources with more rolling fluctuations like PKS 0735+178. The most extreme flares can last for up to 13 years or have peak fluxes of approximately 12 Jy in the observers frame. When the Doppler boosting effect is taken into account, the peak flux of a flare does not depend on the duration of the flare. A rough analysis of the time lags and peak flux evolution indicates that BLO flares in the mm - cm wavelengths are high-peaking, i.e., are in the adiabatic stage. Thus, the results concur with the generalized shock model.
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