AGN in the RASS
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Hamburger Sternwarte - Research

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

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 < 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 νpeak = 16.5. The complete sample shows negative evolution at the 2σ level (< Ve/Va > = 0.42 +- 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.
Distribution of redshifts in the extended HRX-BL Lac sample. The hatched part refers to the complete sample. The plane αox - αro covered by the HRX-BL Lac objects. The points refer to the complete sample, the triangles mark additional objects found within the course of the work. Objects with αro < 0.2 are called radio quiet.

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Last changed  Saturday, 04-March-2006 | D.Engels