Astron. Astrophys. 356, 391-402 (2000)

A time-delay determination from VLA light curves of the CLASS gravitational lens B1600+434

L.V.E. Koopmans ¹, A.G. de Bruyn ^2,1, E. Xanthopoulos ³ and C.D. Fassnacht <sup>4

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 University of Manchester, NRAL Jodrell Bank, Macclesfield, Cheshire SK11 9DL, UK
⁴ National Radio Astronomy Observatory, P.O. Box 0, Socorro, NM 87801, USA

Received 14 June 1999 / Accepted 27 January 2000

Abstract

We present Very Large Array (VLA) 8.5-GHz light curves of the two lens images of the Cosmic Lens All Sky Survey (CLASS) gravitational lens B1600+434. We find a nearly linear decrease of 18-19% in the flux densities of both lens images over a period of eight months (February-October) in 1998. Additionally, the brightest image A shows modulations up to peak-to-peak on scales of days to weeks over a large part of the observing period. Image B varies significantly less on this time scale. We conclude that most of the short-term variability in image A is not intrinsic source variability, but is most likely caused by microlensing in the lens galaxy. The alternative, scintillation by the ionized Galactic ISM, is shown to be implausible based on its strong opposite frequency dependent behavior compared with results from multi-frequency WSRT monitoring observations (Koopmans & de Bruyn 1999).

From these VLA light curves we determine a median time delay between the lens images of d (68%) or d (95%). We use two different methods to derive the time delay; both give the same result within the errors. We estimate an additional systematic error between -8 and +7 d. If the mass distribution of lens galaxy can be described by an isothermal model (Koopmans, de Bruyn & Jackson 1998), this time delay would give a value for the Hubble parameter, H₀= (95% statistical) (systematic) km s^-1 Mpc^-1 (=1 and =0). Similarly, the Modified-Hubble-Profile mass model would give H₀= (95% statistical) (systematic) km s^-1 Mpc^-1. For =0.3 and =0.7, these values increase by 5.4%. We emphasize that the slope of the radial mass profile of the lens-galaxy dark-matter halo in B1600+434 is extremely ill-constrained. Hence, an accurate determination of H₀ from this system is very difficult, if no additional constraints on the mass model are obtained. These values of H₀ should therefore be regarded as indicative.

Once H₀ (from independent methods) and the time delay have been determined with sufficient accuracy, it will prove more worthwhile to constrain the radial mass profile of the dark-matter halo around the edge-on spiral lens galaxy at z=0.4.

Key words: cosmology: distance scale cosmology: gravitational lensing galaxies: halos galaxies: quasars: individual: B1600+434 galaxies: structure

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

Correspondence to: leon@astro.rug.nl

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Contents

• 1. Introduction
• 2. Data and reduction
  • 2.1. Observations
  • 2.2. Calibration
  • 2.3. Imaging and model fitting
  • 2.4. Flux calibration
  • 2.5. Light curves of B1600+434
  • 2.6. External variability
  • 2.7. Error analysis
• 3. Analysis
  • 3.1. The time delay
    • 3.1.1. Minimum dispersion method
    • 3.1.2. Median time delay and statistical error range
    • 3.1.3. Systematic uncertainties in the time delay
    • 3.1.4. The PRH method
  • 3.2. The Hubble parameter and slope of the radial mass profile of the lens-galaxy dark-matter halo
• 4. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000
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