L. V. E. Koopmans

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LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometer constructed in the north of the Netherlands and across europe. Utilizing a novel phased-array design, LOFAR covers the largely unexplored low-frequency range from 10–240 MHz and provides a number of unique observing capabilities. Spreading out from a core located near the village of(More)
Measurement of the time delay between multiple images of a gravitational lens system is potentially an accurate method of determining the Hubble constant over cosmological distances. One of the most promising candidates for an application of this technique is the system B0218+357 which was found in the Jodrell Bank/VLA Astrometric Survey (JVAS). This system(More)
We present the results of a program to monitor the four-image gravitational lens B1608+656 with the VLA. The system was observed over a seven month period from 1996 October to 1997 May. The 64 epochs of observation have an average spacing of 3.6 d. The light curves of the four images of the background source show that the flux density of the background(More)
A single-screen model of the gravitational lens system 2016+112 is proposed, that explains recent Hubble Space Telescope (HST) infrared (NICMOS–F160W) observations and new high-resolution European VLBI Network (EVN) 5–GHz radio observations, presented in this paper. In particular, we find that a massive 'dark' structure at the lens position, previously(More)
We present a refined gravitational lens model of the four-image lens system B1608+656 based on new and improved observational constraints: (i) the three independent time-delays and flux-ratios from Very Large Array (VLA) observations, (ii) the radio-image positions from Very Large Baseline Array (VLBA) observations, (iii) the shape of the deconvolved(More)
The existence of flux-ratio anomalies between fold and cusp images in galaxy-scale strong-lens systems has led to an interpretation based on the presence of a high massfraction of cold-dark-matter (CDM) substructures around galaxies, as predicted by numerical N-body simulations. These substructures can cause large perturbations of the image magnifications,(More)
We derive constraints on cosmological parameters and the properties of the lensing galaxies from gravitational lens statistics based on the final Cosmic Lens All Sky Survey data. For a flat universe with a classical cosmological constant, we find that the present matter fraction of the critical density is Omega(m)=0.31(+0.27)(-0.14) (68%)+0.12-0.10 (syst).(More)
The mass function of dwarf satellite galaxies that are observed around Local Group galaxies differs substantially from simulations based on cold dark matter: the simulations predict many more dwarf galaxies than are seen. The Local Group, however, may be anomalous in this regard. A massive dark satellite in an early-type lens galaxy at a redshift of 0.222(More)
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 11%(More)
Strong gravitational lens systems with measured time delays between the multiple images provide a method for measuring the “time-delay distance” to the lens, and thus the Hubble constant. We present a Bayesian analysis of the strong gravitational lens system B1608+656, incorporating (i) new, deep Hubble Space Telescope (HST) observations, (ii) a new(More)