We study the cosmological evolution of BL Lac objects by applying the Ve/Va analysis to a new sample of 55 objects, presented for the first time in this paper. This sample has been selected from the 239 sources with the brightest X-ray flux (≥4 × 10-13 ergs s-1 cm-2) and relatively bright optical counterparts (B ≤ 20.5) among the ~1600 objects included in the REX survey. The uniform distribution of the Ve/Va values found in the sample suggests that BL Lac objects are not affected by a strong cosmological evolution, in contrast with the behavior observed in the emission-line active galactic nuclei. The Ve/Va analysis applied to the subsample of the high-energy-peaked BL Lac objects (HBLs) does not reveal a significant departure from a uniform distribution. This result suggests either that the cosmological evolution is less extreme than that previously found in other samples or that it is carried out only by a minority of objects, namely, by the most extreme HBLs not selected in our sample.
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