EDP Sciences
Free access
Volume 401, Number 3, April III 2003
Page(s) 927 - 938
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20030184

A&A 401, 927-938 (2003)
DOI: 10.1051/0004-6361:20030184

The HRX-BL Lac sample - Evolution of BL Lac objects

V. Beckmann1, 2, 3, D. Engels1, N. Bade1 and O. Wucknitz4

1  Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
2  INTEGRAL Science Data Centre, Chemin d'Écogia 16, 1290 Versoix, Switzerland
3  Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
4  Universität Potsdam, Institut für Physik, Am Neuen Palais 10, 14469 Potsdam, Germany

(Received 8 April 2002 / Accepted 12 February 2003)

The unification of X-ray and radio selected BL Lacs has been an outstanding problem in the blazar research in the past years. Recent investigations have shown that the gap between the two classes can be filled with intermediate objects and that apparently all differences can be explained by mutual shifts of the peak frequencies of the synchrotron and inverse Compton component of the emission. We study the consequences of this scheme using a new sample of X-ray selected BL Lac objects comprising 104 objects with z<0.9 and a mean redshift $\bar{z} = 0.34$. 77 BL Lacs, of which the redshift could be determined for 64 (83%) objects, form a complete sample. The new data could not confirm our earlier result, drawn from a subsample, that the negative evolution vanishes below a synchrotron peak frequency $\log \nu_{\rm peak} = 16.5$. The complete sample shows negative evolution at the 2 $\sigma$ level ( $\langle V_{\rm
e}/V_{\rm a} \rangle = 0.42 \pm 0.04$ ). We conclude that the observed properties of the HRX BL Lac sample show typical behaviour for X-ray selected BL Lacs. They support an evolutionary model, in which flat-spectrum radio quasars (FSRQ) with high energetic jets evolve towards low frequency peaked (mostly radio-selected) BL Lac objects and later on to high frequency peaked (mostly X-ray selected) BL Lacs.

Key words: BL Lacertae objects: general -- X-rays: galaxies

Offprint request: V. Beckmann, Volker.Beckmann@obs.unige.ch

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© ESO 2003