The Connection between X-ray- and Radio-Selected BL Lacertae Objects

We explain the properties of X-ray selected BL Lacertae objects, under the assumption that they constitute the small minority of the BL Lac population with energy cutoff located in the UV/X-ray band, as suggested by their multifrequency spectra. In particular, we derive their X-ray luminosity function, log N-log S, and radio flux distribution starting from radio selected BL Lacs using a method that does not require any free parameters. Our hypothesis is at variance with the most common explanation for the existence of two classes of BL Lacs, namely that X-ray selected objects are viewed significantly off the beaming axis while radio-selected ones have their jets more aligned with the line of sight. In this case, in fact, X-ray selected sources should be intrinsically more numerous than objects selected in the radio band at a fixed value of some isotropic luminosity such as that of the diffuse radio emission. While also accounting for most of the properties of BL Lacs detected in current X-ray surveys, this latter scenario would predict their radio emission to be dominated by the extended (unbeamed) component, in contrast with observations. We also address the question of the negative evolution apparently exhibited by X-ray selected BL Lacs, showing that neither recognition problems nor a break in the X-ray spectrum are likely to be responsible for it. Our hypothesis explains in a simple way the bimodal distribution of BL Lacs in the alpha_ro - alpha_ ox diagram. It also makes specific predictions for the X-ray, radio, and optical properties of BL Lacs which will be tested by future deep X-ray surveys.