2QZJ 215454.3-305654: a radio-quiet BL Lacertae object or lineless quasi-stellar object?

High signal-to-noise ratio spectroscopy has established a redshift of z = 0.494 for the source 2QZJ 215454.3-305654, originally selected from the 2dF/6dF QSO Redshift Surveys as one of 45 candidate BL Lacertae (BL Lac) objects displaying a featureless continuum at optical wavelengths. Radio observations using the Australia Telescope Compact Array at 1.4 GHz place a 3σ upper limit on the radio flux density of the object of ∼0.14 mJy. The radio-to-optical flux ratio of this object is thus more than seven times lower than the lowest such ratio observed in BL Lac objects. While the optical properties of 2QZJ 215454.3-305654 are consistent with a BL Lac identification, the lack of radio and/or X-ray emission is not. It is uncertain whether this object is an active galactic nucleus dominated by optical continuum emission from an accretion disc, or is similar to a BL Lac object with optical non-thermal emission from a relativistic jet.