424 research outputs found
Simulation of microlensing lightcurves by combining contouring and rayshooting
The contouring methods described by Lewis et al. (1993) and Witt (1993) are
very efficient and elegant for obtaining the magnification of a point source
moving along a straight track in the source plane. The method is, however, not
very efficient for extended sources, because the amplification needs to be
computed for numerous parallel tracks and then convolved with the source
profile. Rayshooting is an efficient algorithm for relatively large sources,
but the computing time increases with the inverse of the source area for a
given noise level. This poster presents a hybrid method, using the contouring
method in order to find only those parts of the lens area that contribute to
the light curve through the rayshooting. Calculations show that this method has
the potential to be -- times more efficient than crude rayshooting
techniques.Comment: 2 pages, no figures. Uses crckapb.sty. To appear in the Proceedings
of the IAU Symposium 173: ``Astrophysical Applications of Gravitational
Lensing'', Kluwer Academic Publishers, Eds.: C. S. Kochanek and J. N. Hewitt.
Also available, with addditional information, through
http://www.uio.no/~steinhh/index.htm
Separating intrinsic and microlensing variability using parallax measurements
In gravitational lens systems with 3 or more resolved images of a quasar, the
intrinsic variability may be unambiguously separated from the microlensing
variability through parallax measurements from 3 observers when there is no
relative motion of the lens masses (Refsdal 1993). In systems with fewer than 3
resolved images, however, this separation is not straightforward. A general
approach that may be used for this purpose is presented. For simplicity, only
the one-dimensional case is considered in detail: Given a well-sampled time
series of the observed flux at two points in space with a known separation,
choosing a velocity of the observers perpendicular to the line of
sight determines the microlensing magnification history, and thereby also the
intrinsic variability. The velocity is chosen by minimizing some measure
() of the residual intrinsic variability. In many cases this gives a
close approximation to the true magnification. In cases where the relative
motion of the lensing point masses is important, only a partial separation will
be possible.Comment: 2 pages LaTeX, no figures. Uses crckapb.sty. To appear in the
Proceedings of the IAU Symposium 173: ``Astrophysical Applications of
Gravitational Lensing'', Kluwer Academic Publishers, Eds.: C. S. Kochanek and
J. N. Hewitt. Also available, with additional information, through
http://www.uio.no/~steinhh/index.htm
The microlensing events in Q2237+0305A: No case against small masses/large sources
It is demonstrated that the 1988-90 microlensing events in image A of
Q2237+0305 reported by Racine (1992) do not exclude microlensing models with
very low average mass, making the source radius larger than the projected
Einstein radius (Refsdal and Stabell 1991, 1993). This is contrary to
what has been claimed by Witt and Mao (1994). Since these events are the best
resolved microlensing events recorded in Q2237+0305, further work should not
exclude the possibility of a large source when interpreting lightcurve data.Comment: 2 pages, 1 figure. Uses crckapb.sty. To appear in the Proceedings of
the IAU Symposium 173: ``Astrophysical Applications of Gravitational
Lensing'', Kluwer Academic Publishers, Eds.: C. S. Kochanek and J. N. Hewitt.
Also available through http://www.uio.no/~steinhh/index.htm
EUV Sunspot Plumes Observed with SOHO
Bright EUV sunspot plumes have been observed in five out of nine sunspot
regions with the Coronal Diagnostic Spectrometer -- CDS on SOHO. In the other
four regions the brightest line emissions may appear inside the sunspot but are
mainly concentrated in small regions outside the sunspot areas. These results
are in contrast to those obtained during the Solar Maximum Mission, but are
compatible with the Skylab mission results. The present observations show that
sunspot plumes are formed in the upper part of the transition region, occur
both in magnetic unipolar-- and bipolar regions, and may extend from the umbra
into the penumbra.Comment: 8 pages, 3 figures, to be published in ApJ Letter
Dirac theory within the Standard-Model Extension
The modified Dirac equation in the Lorentz-violating Standard-Model Extension
(SME) is considered. Within this framework, the construction of a hermitian
Hamiltonian to all orders in the Lorentz-breaking parameters is investigated,
discrete symmetries and the first-order roots of the dispersion relation are
determined, and various properties of the eigenspinors are discussed.Comment: 11 pages REVTe
Wave propagation in linear electrodynamics
The Fresnel equation governing the propagation of electromagnetic waves for
the most general linear constitutive law is derived. The wave normals are found
to lie, in general, on a fourth order surface. When the constitutive
coefficients satisfy the so-called reciprocity or closure relation, one can
define a duality operator on the space of the two-forms. We prove that the
closure relation is a sufficient condition for the reduction of the fourth
order surface to the familiar second order light cone structure. We finally
study whether this condition is also necessary.Comment: 13 pages. Phys. Rev. D, to appea
Dynamical Lorentz simmetry breaking from 3+1 Axion-Wess-Zumino model
We study the renormalizable abelian vector-field models in the presence of
the Wess-Zumino interaction with the pseudoscalar matter. The renormalizability
is achieved by supplementing the standard kinetic term of vector fields with
higher derivatives. The appearance of fourth power of momentum in the
vector-field propagator leads to the super-renormalizable theory in which the
-function, the vector-field renormalization constant and the anomalous
mass dimension are calculated exactly. It is shown that this model has the
infrared stable fixed point and its low-energy limit is non-trivial. The
modified effective potential for the pseudoscalar matter leads to the possible
occurrence of dynamical breaking of the Lorentz symmetry. This phenomenon is
related to the modification of Electrodynamics by means of the Chern-Simons
(CS) interaction polarized along a constant CS vector. Its presence makes the
vacuum optically active that has been recently estimated from astrophysical
data. We examine two possibilities for the CS vector to be time-like or
space-like, under the assumption that it originates from v.e.v. of some
pseudoscalar matter and show that only the latter one is consistent in the
framework of the AWZ model, because a time-like CS vector makes the vacuum
unstable under pairs creation of tachyonic photon modes with the finite vacuum
decay rate.Comment: 33 pages, no Figures, Plain TeX, submitted to Phys. Rev.
Model-Independent Comparisons of Pulsar Timings to Scalar-Tensor Gravity
Observations of pulsar timing provide strong constraints on scalar-tensor
theories of gravity, but these constraints are traditionally quoted as limits
on the microscopic parameters (like the Brans-Dicke coupling, for example) that
govern the strength of scalar-matter couplings at the particle level in
particular models. Here we present fits to timing data for several pulsars
directly in terms of the phenomenological couplings (masses, scalar charges,
moment of inertia sensitivities and so on) of the stars involved, rather than
to the more microscopic parameters of a specific model. For instance, for the
double pulsar PSR J0737-3039A/B we find at the 68% confidence level that the
masses are bounded by 1.28 < m_A/m_sun < 1.34 and 1.19 < m_B/m_sun < 1.25,
while the scalar-charge to mass ratios satisfy |a_A| < 0.21, |a_B| < 0.21 and
|a_B - a_A| < 0.002$. These constraints are independent of the details of the
scalar tensor model involved, and of assumptions about the stellar equations of
state. Our fits can be used to constrain a broad class of scalar tensor
theories by computing the fit quantities as functions of the microscopic
parameters in any particular model. For the Brans-Dicke and quasi-Brans-Dicke
models, the constraints obtained in this manner are consistent with those
quoted in the literature.Comment: 19 pages, 7 figure
Effective dissipative dynamics for polarized photons
In the framework of open quantum systems, the propagation of polarized
photons can be effectively described using quantum dynamical semigroups. These
extended time-evolutions induce irreversibility and dissipation. Planned, high
sensitive experiments, both in the laboratory and in space, will be able to put
stringent bounds on these non-standard effects.Comment: 15 pages, plain-TeX, no figure
Gravity, Lorentz Violation, and the Standard Model
The role of the gravitational sector in the Lorentz- and CPT-violating
Standard-Model Extension (SME) is studied. A framework is developed for
addressing this topic in the context of Riemann-Cartan spacetimes, which
include as limiting cases the usual Riemann and Minkowski geometries. The
methodology is first illustrated in the context of the QED extension in a
Riemann-Cartan background. The full SME in this background is then considered,
and the leading-order terms in the SME action involving operators of mass
dimension three and four are constructed. The incorporation of arbitrary
Lorentz and CPT violation into general relativity and other theories of gravity
based on Riemann-Cartan geometries is discussed. The dominant terms in the
effective low-energy action for the gravitational sector are provided, thereby
completing the formulation of the leading-order terms in the SME with gravity.
Explicit Lorentz symmetry breaking is found to be incompatible with generic
Riemann-Cartan geometries, but spontaneous Lorentz breaking evades this
difficulty.Comment: 21 pages REVTeX, references added, accepted in Physical Review
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