2,912 research outputs found

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

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% 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 47^{+5}_{-6} d (68%)
or 47^{+12}_{-9} 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, H_0=57^{+14}_{-11}
(95% statistical) ^{+26}_{-15} (systematic) km/s/Mpc (Omega_m=1 and
Omega_Lambda=0). Similarly, the Modified-Hubble-Profile mass model would give
H_0=74^{+18}_{-15} (95% statistical) ^{+22}_{-22} (systematic) km/s/Mpc. For
Omega_m=0.3 and Omega_Lambda=0.7, these values increase by 5.4%. ... (ABRIDGED)Comment: 14 pages, 6 figures, accepted for publication in Astronomy &
Astrophysics (Figs 1 and 3 with degraded resolution

### VLA 8.4-GHz monitoring observations of the CLASS gravitational lens B1933+503

The complex ten-component gravitational lens system B1933+503 has been
monitored with the VLA during the period February to June 1998 with a view to
measuring the time delay between the four compact components and hence to
determine the Hubble parameter. Here we present the results of an `A'
configuration 8.4-GHz monitoring campaign which consists of 37 epochs with an
average spacing of 2.8 days. The data have yielded light curves for the four
flat-spectrum radio components (components 1, 3, 4 and 6). We observe only
small flux density changes in the four flat-spectrum components which we do not
believe are predominantly intrinsic to the source. Therefore the variations do
not allow us to determine the independent time delays in this system. However,
the data do allow us to accurately determine the flux density ratios between
the four flat-spectrum components. These will prove important as modelling
constraints and could prove crucial in future monitoring observations should
these data show only a monotonic increase or decrease in the flux densities of
the flat-spectrum components.Comment: Accepted for publication in MNRAS. 5 pages, 2 included PostScript
figure

### A determination of H_0 with the CLASS gravitational lens B1608+656: II. Mass models and the Hubble constant from lensing

EDITED FROM PAPER: We present mass models of the four-image gravitational
lens system B1608+656. A mass model for the lens galaxies has been determined
that reproduces the image positions, two out of three flux-density ratios and
the model time delays.
Using the time delays determined by Fassnacht et al. (1999a), we find that
the best isothermal mass model gives H_0=59^{+7}_{-6} km/s/Mpc for Omega_m=1
and Omega_l=0.0, or H_0=(65-63)^{+7}_{-6} km/s/Mpc for Omega_m=0.3 and Omega_l
= 0.0-0.7 (95.4% statistical confidence). A systematic error of +/-15 km/s/Mpc
is estimated.
This cosmological determination of H_0 agrees well with determinations from
three other gravitational lens systems (i.e. B0218+357, Q0957+561 and
PKS1830-211), SNe Ia, the S-Z effect and local determinations. The current
agreement on H_0 from four out of five gravitational lens systems (i)
emphasizes the reliability of its determination from isolated gravitational
lens systems and (ii) suggests that a close-to-isothermal mass profile can
describe disk galaxies, ellipticals and central cluster ellipticals.
The average of H_0 from B0218+357, Q0957+561, B1608+656 and PKS1830-211,
gives H_0(GL)=69 +/-7 km/s/Mpc for a flat universe with Omega_m=1 or H_0(GL)=74
+/-8 km/s/Mpc for Omega_m=0.3 and Omega_l=0.0-0.7. When including PG1115+080,
these values decrease to 64 +/-11 km/s/Mpc and 68 +/-13 km/s/Mpc (2-sigma
errors), respectively.Comment: Accepted for publication in ApJ. 34 pages, 4 figure

### The Hubble Constant from the Gravitational Lens B1608+656

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 VLA observations, (ii) the
radio-image positions from VLBA observations, (iii) the shape of the
deconvolved Einstein Ring from optical and infrared HST images, (iv) the
extinction-corrected lens-galaxy centroids and structural parameters, and (v) a
stellar velocity dispersion, sigma_ap=247+-35 km/s, of the primary lens galaxy
(G1), obtained from an echelle spectrum taken with the Keck--II telescope. The
lens mass model consists of two elliptical mass distributions with power-law
density profiles and an external shear, totaling 22 free parameters, including
the density slopes which are the key parameters to determine the value of H_0
from lens time delays. This has required the development of a new lens code
that is highly optimized for speed. The minimum-chi^2 model reproduces all
observations very well, including the stellar velocity dispersion and the shape
of the Einstein Ring. A combined gravitational-lens and stellar dynamical
analysis leads to a value of the Hubble Constant of H_0=75(+7/-6) km/s/Mpc (68
percent CL; Omega_m=0.3, Omega_Lambda=0.7. The non-linear error analysis
includes correlations between all free parameters, in particular the density
slopes of G1 and G2, yielding an accurate determination of the random error on
H_0. The lens galaxy G1 is ~5 times more massive than the secondary lens galaxy
(G2), and has a mass density slope of gamma_G1=2.03(+0.14/-0.14) +- 0.03 (68
percent CL) for rho~r^-gamma', very close to isothermal (gamma'=2). (Abridged)Comment: 17 pages, 6 figures, 5 tables; revised version with correct fig.6 and
clarified text based on referee report; conclusions unchange

### Measuring gravitational lens time delays using low-resolution radio monitoring observations

Obtaining lensing time delay measurements requires long-term monitoring
campaigns with a high enough resolution (< 1 arcsec) to separate the multiple
images. In the radio, a limited number of high-resolution interferometer arrays
make these observations difficult to schedule. To overcome this problem, we
propose a technique for measuring gravitational time delays which relies on
monitoring the total flux density with low-resolution but high-sensitivity
radio telescopes to follow the variation of the brighter image. This is then
used to trigger high-resolution observations in optimal numbers which then
reveal the variation in the fainter image. We present simulations to assess the
efficiency of this method together with a pilot project observing radio lens
systems with the Westerbork Synthesis Radio Telescope (WSRT) to trigger Very
Large Array (VLA) observations. This new method is promising for measuring time
delays because it uses relatively small amounts of time on high-resolution
telescopes. This will be important because instruments that have high
sensitivity but limited resolution, together with an optimum usage of followup
high-resolution observations from appropriate radio telescopes may in the
future be useful for gravitational lensing time delay measurements by means of
this new method.Comment: 10 pages, 7 figures, accepted by MNRA

### The Cosmic Lens All-Sky Survey parent population - I. Sample selection and number counts

We present the selection of the Jodrell Bank Flat-spectrum (JBF) radio source
sample, which is designed to reduce the uncertainties in the Cosmic Lens
All-Sky Survey (CLASS) gravitational lensing statistics arising from the lack
of knowledge about the parent population luminosity function. From observations
at 4.86 GHz with the Very Large Array, we have selected a sample of 117
flat-spectrum radio sources with flux densities greater than 5 mJy. These
sources were selected in a similar manner to the CLASS complete sample and are
therefore representative of the parent population at low flux densities. The
vast majority (~90 per cent) of the JBF sample are found to be compact on the
arcsecond scales probed here and show little evidence of any extended radio jet
emission. Using the JBF and CLASS complete samples we find the differential
number counts slope of the parent population above and below the CLASS 30 mJy
flux density limit to be -2.07+/-0.02 and -1.96+/-0.12, respectively.Comment: 10 pages, 4 figures, accepted for publication in MNRA

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