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Astron. Astrophys. 334, 459-472 (1998)

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On the evolutionary behaviour of BL Lac objects *

N. Bade 1, V. Beckmann 1, N.G. Douglas 2, P.D. Barthel 2, D. Engels 1, L. Cordis 1, P. Nass 3 and W. Voges 3

1 Hamburger Sternwarte, Gojenbergsweg 112, D-21029 Hamburg, Germany
2 Kapteyn Astronomical Institute, P.O.Box 800, 9700 AV Groningen, The Netherlands
3 MPI für Extraterrestrische Physik, D-85740 Garching, Germany

Received 12 November 1997 / Accepted 18 February 1998


We present a new well defined sample of BL Lac objects selected from the ROSAT All-Sky Survey (RASS). The sample consists of 39 objects with 35 forming a flux limited sample down to [FORMULA] ; redshifts are known for 33 objects (and 31 of the complete sample). X-ray spectral properties were determined for each object individually with the RASS data. The luminosity function of RASS selected BL Lac objects is compatible with results provided by objects selected with the Einstein observatory, but the RASS selected sample contains objects with luminosities at least tenfold higher. Our analysis confirms the negative evolution for X-ray selected BL Lac objects found in a sample by the Einstein observatory, the parameterization provides similar results. A subdivision of the sample into halves according to the X-ray to optical flux ratio yielded unexpected results. The extremely X-ray dominated objects have higher redshifts and X-ray luminosities and only this subgroup shows clear signs of strong negative evolution. The evolutionary behaviour of objects with an intermediate spectral energy distribution between X-ray and radio dominated is compatible with no evolution at all. Consequences for unified schemes of X-ray and radio selected BL Lac objects are discussed. We suggest that the intermediate BL Lac objects are the basic BL Lac population. The distinction between the two subgroups can be explained if extremely X-ray dominated BL Lac objects are observed in a state of enhanced X-ray activity.

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

* Based on observations from the German-Spanish Astronomical Center, Calar Alto, operated jointly by the Max-Planck-Institut für Astronomie, Heidelberg, and the Spanish National Commission for Astronomy, and from the William Herschel Telescope operated on the island of La Palma by the Royal Greenwich Observatory in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias

Send offprint requests to: N. Bade, (nbade@hs.uni-hamburg.de)

© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998