1999MNRAS.307..403S

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 cut-off BL Lac objects (HBLs), 1 low-energy cut-off BL Lac object (LBL) and 2 `intermediate' 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 (α-_5–230=-0.29±0.15) are slightly steeper than those of the LBLs (α-_5–230=-0.19±0.14). A correlation exists between α_5–230 and 230GHz luminosity. 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 that relate BL Lac objects to Fanaroff-Riley class I radio galaxies.

The HBLs have significantly flatter submillimetre-X-ray spectral indices (α-_230–X=-0.68±0.07) than the LBLs (α-_230–X=-1.08±0.06) although the two intermediate sources also have intermediate values of α_230–X~-0.9. It is argued that this difference cannot be explained entirely by the viewing-angle hypothesis and requires a difference in physical-source parameters. The α_230–X values for the HBLs are close to the canonical value found for large samples of radio sources and thus suggest that synchrotron radiation is the mechanism that produces the X-ray emission. As suggested by Padovani & Giommi, the inverse-Compton mechanism is likely to dominate in the LBLs requiring the synchrotron spectra of these sources to steepen or cut off at lower frequencies than those of the HBLs.