SIMBAD references

We study the cosmological evolution of BL Lac objects by applying the V_e/V_a 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 (≥4x10^–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 V_e/V_a 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 V_e/V_a 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.