Variations in the broad-band spectra of BL Lac objects: millimetre observations of an X-ray-selected sample

Observations at millimetre wavelengths are presented for a representative sample of 22 X-ray-selected BL Lac objects (XBLs). This sample comprises 19 High-energy cutoff BL Lacs objects (HBLs), 1 Low-energy cutoff BL Lac object (LBL) and 2 ‘inter- mediate’ sources. Data for LBLs, which are mostly radio selected BL Lac objects (RBLs) are taken from the literature. It is shown that the radio–millimetre spectral indices of HBLs (¯α5−230 = −0.29 ± 0.15) are slightly steeper than those of the LBLs (¯α5−230 = −0.19 ± 0.14). A correlation exists between α5−230 and 230 GHz luminos- ity. While this correlation could be an artefact of comparing two populations of BL Lac objects with intrinsically different radio properties it is also consistent with the predictions of existing unified schemes which relate BL Lac objects to Fanaroff-Riley class I radio galaxies.