EDP Sciences
Free access
Volume 445, Number 2, January II 2006
Page(s) 441 - 450
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20053316

A&A 445, 441-450 (2006)
DOI: 10.1051/0004-6361:20053316

Spectral energy distributions of a large sample of BL Lacertae objects

E. Nieppola1, M. Tornikoski2 and E. Valtaoja1, 3

1  Tuorla Observatory, Väisäläntie 20, 21500 Piikkiö, Finland
    e-mail: eni@kurp.hut.fi
2  Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
3  Dept. of Physical Sciences, University of Turku, 20100 Turku, Finland

(Received 26 April 2005 / Accepted 26 August 2005)

We have collected a large amount of multifrequency data for objects in the Metsähovi Radio Observatory BL Lacertae sample and computed their spectral energy distributions (SED) in the log $\nu$ - log $\
\nu F$ - representation. 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, $\nu_{\rm peak}$. 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 $\nu_{\rm peak}$.

The range of $\nu_{\rm peak}$ in our study was considerably extended compared to previous studies. There were 22 objects for which log $\nu_{\rm peak}>$19. The data shows that at 5 GHz, 37 GHz, and 5500 ${\rm\AA}$, there is negative correlation between luminosity and $\nu_{\rm peak}$, whereas in X-rays the correlation turns slightly positive. There is no significant correlation between source luminosity at synchrotron peak and $\nu_{\rm peak}$. Several low radio luminosity-low energy peaked BL Lacs were found. The negative correlation between broad band spectral indices and $\nu_{\rm peak}$ is also significant, although there is substantial scatter. Therefore we find that neither $\alpha_{\rm rx}$ nor  $\alpha_{\rm ro}$ 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.

Key words: galaxies: active -- BL Lacertae objects: general -- radiation mechanisms: non-thermal

SIMBAD Objects

© ESO 2005

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