CLASS B0827+525: 'Dark lens' or binary radio-loud quasar?
We present radio, optical, near-infrared and spectroscopic 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 ~3%, although the presence of similar substructure in both component might reduce this. We discuss two 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) ≤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.
Additional InformationThe authors would like to thank Lee Armus and David Hogg for obtaining a Keck NIRC K–band image. LVEK and AGdeB acknowledge the support from an NWO program subsidy (grant number 781-76-101). This research was supported in part by the European Commission, TMR Programme, Research Network Contract ERBFMRXCT96-0034 'CERES'. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The Westerbork Synthesis Radio Telescope (WSRT) is operated by the Netherlands Foundation for Research in Astronomy (ASTRON) with the financial support from the Netherlands Organization for Scientific Research (NWO). MERLIN is a national UK facility operated by the University of Manchester on behalf of PPARC. This research used observations with the Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by Associated Universities for Research in Astronomy Inc. under NASA contract NAS5-26555.
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