209 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
Comparative analysis of time-frequency methods estimating the time-varying microstructure of sleep EEG spindles
Proceedings of the Information Technology Applications in Biomedicine, Ioannina - Epirus, Greece, October 26-28, 2006Parameter estimation for an assumed sleep
EEG spindle model (AM-FM signal) is performed by using four time-frequency analysis methods. Results from simulated as well as from real data are presented. In
simulated data, the Hilbert Transform-based method has the lowest average percentage error but produces considerable signal distortion. The Complex Demodulation
and the Matching Pursuit-based methods have error rates below 10%, but the Matching Pursuit-based method produces considerable signal distortion as well. The Wavelet
Transform-based method has the poorest performance. In real data, all methods produce reasonable parameter values.
However, the Hilbert Transform and the Matching Pursuitbased methods may not be applicable for sleep spindles shorter than about 0.8 sec. Matching Pursuit-based curve
fitting is utilized as part of the parameter estimation process
Matching of analytical and numerical solutions for neutron stars of arbitrary rotation
We demonstrate the results of an attempt to match the two-soliton analytical
solution with the numerically produced solutions of the Einstein field
equations, that describe the spacetime exterior of rotating neutron stars, for
arbitrary rotation. The matching procedure is performed by equating the first
four multipole moments of the analytical solution to the multipole moments of
the numerical one. We then argue that in order to check the effectiveness of
the matching of the analytical with the numerical solution we should compare
the metric components, the radius of the innermost stable circular orbit
(), the rotation frequency and the
epicyclic frequencies . Finally we present some
results of the comparison.Comment: Contribution at the 13th Conference on Recent Developments in Gravity
(NEB XIII), corrected typo in of eq. 5 of the published versio
Dipole Perturbations of the Reissner-Nordstrom Solution: The Polar Case
The formalism developed by Chandrasekhar for the linear polar perturbations
of the Reissner-Nordstrom solution is generalized to include the case of dipole
(l=1) perturbations. Then, the perturbed metric coefficients and components of
the Maxwell tensor are computed.Comment: 16 pages, LaTeX, no figures. Submitted for publication in Physical
Review
Chaos in black holes surrounded by gravitational waves
The occurrence of chaos for test particles moving around Schwarzschild black
holes perturbed by a special class of gravitational waves is studied in the
context of the Melnikov method. The explicit integration of the equations of
motion for the homoclinic orbit is used to reduce the application of this
method to the study of simple graphics.Comment: 15 pages, LaTex
Conformally dressed black hole in 2+1 dimensions
A three dimensional black hole solution of Einstein equations with negative
cosmological constant coupled to a conformal scalar field is given. The
solution is static, circularly symmetric, asymptotically anti-de Sitter and
nonperturbative in the conformal field. The curvature tensor is singular at the
origin while the scalar field is regular everywhere. The condition that the
Euclidean geometry be regular at the horizon fixes the temperature to be
. Using the Hamiltonian formulation including
boundary terms of the Euclidean action, the entropy is found to be
of the standard value (), and in agreement with
the first law of thermodynamics.Comment: LaTeX ,RevTeX, 13pages, no figure
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