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| Paper: |
Evolutionary Behaviour of BL Lac Objects |
| Volume: |
159, BL Lac Phenomenon |
| Page: |
493 |
| Authors: |
Beckmann, Volker |
| Abstract: |
We present a new well defined sample of BL Lac objects selected from the ROSAT All-Sky Survey. The sample is based on an area of about 4000deg^{-2} at the northern hemisphere. X-ray spectral properties were determined for each object individually with the RASS data. 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. The distinction between the two subgroups can be explained if extreme X-ray dominated BL Lac objects are observed in a state of enhanced X-ray activity. |
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