Astron. Astrophys. 361, 815-822 (2000)

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

L.V.E. Koopmans ^3,5, A.G. de Bruyn ^2,1, C.D. Fassnacht ^3,4, D.R. Marlow ^5,6, D. Rusin ⁶, R.D. Blandford ³, I.W.A. Browne ⁵, P. Helbig ^5,1, N. Jackson ⁵, S.T. Myers ^6,4, T.J. Pearson ³, A.C.S. Readhead ³, P.N. Wilkinson ⁵, E. Xanthopoulos ⁵ and H. Hoekstra <sup>1

1</sup> Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands
² NFRA, P.O. Box 2, 7990 AA Dwingeloo, The Netherlands
³ California Institute of Technology, Pasadena, CA 91125, USA
⁴ NRAO, P.O.Box O, Socorro, NM 87801, USA
⁵ University of Manchester, NRAL Jodrell Bank, Macclesfield, Cheshire SK11 9DL, England, UK
⁶ University of Pennsylvania, Department of Physics, Philadelphia, PA 19104, USA

Received 16 December 1999 / Accepted 7 July 2000

Abstract

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 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 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.

Key words: cosmology: gravitational lensing cosmology: dark matter galaxies: quasars: individual: B0827+525

Send offprint requests to: L.V.E. Koopmans

Correspondence to: leon@jb.man.ac.uk

SIMBAD Objects

Contents

• 1. Introduction
• 2. Observations
  • 2.1. Radio observations
  • 2.2. Optical and near-infrared observations
  • 2.3. Spectroscopic observations
• 3. Analysis
  • 3.1. The `dark-lens' hypothesis
  • 3.2. The binary-quasar hypothesis
• 4. Discussion & conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: October 10, 2000
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