CLASS B0827+525: 'Dark lens' or binary radio-loud quasar?

We present radio, optical, near-infrared and spec troscopic observations of the source B0827+525. We consider this source as the best candidate from the Cosmic Lens All-Sky Survey (CLASS) for a 'dark-lens' system or binary radio-loud quasar. The system consists of two radio components with somewhat different spectral indices, separated by 2.815 arcsec. VLBA observations show that each component has substructure on a scale of a few mas. A deep K-band exposure with the W.M. Keck-II Telescope reveals emission near both radio components. The K-band emission of the weaker radio component appears extended, whereas the emission from the brighter radio component is consistent with a point source. Hubble Space Telescope F160W-band observations with the NICMOS instrument confirms this. A redshift of 2.064 is found for the brighter component, using the LRIS instrument on the W.M. Keck-II Telescope. The probability that B0827+525 consists of two unrelated compact flat-spectrum radio sources is similar to3%, although the presence of similar substructure in both component might reduce this. We discuss two alternative scenarios to explain this system: (i) CLASS B0827+525 is a 'dark-lens' system or (ii) B0827+525 is a binary radio-loud quasar. B0827+525 has met all criteria that thus far have in 100% of the cases confirmed a source as an indisputable gravitational lens system. Despite this, no lens galaxy has been detected with m(F160W)less than or equal to 23 mag. Hence, we might have found the first binary radio-loud quasar. At this moment, however, we feel that the 'dark-lens' hypothesis cannot yet be fully excluded.