1,709 research outputs found
Light's Bending Angle due to Black Holes: From the Photon Sphere to Infinity
The bending angle of light is a central quantity in the theory of
gravitational lensing. We develop an analytical perturbation framework for
calculating the bending angle of light rays lensed by a Schwarzschild black
hole. Using a perturbation parameter given in terms of the gravitational radius
of the black hole and the light ray's impact parameter, we determine an
invariant series for the strong-deflection bending angle that extends beyond
the standard logarithmic deflection term used in the literature. In the
process, we discovered an improvement to the standard logarithmic deflection
term. Our perturbation framework is also used to derive as a consistency check,
the recently found weak deflection bending angle series. We also reformulate
the latter series in terms of a more natural invariant perturbation parameter,
one that smoothly transitions between the weak and strong deflection series. We
then compare our invariant strong deflection bending-angle series with the
numerically integrated exact formal bending angle expression, and find less
than 1% discrepancy for light rays as far out as twice the critical impact
parameter. The paper concludes by showing that the strong and weak deflection
bending angle series together provide an approximation that is within 1% of the
exact bending angle value for light rays traversing anywhere between the photon
sphere and infinity.Comment: 22 pages, 5 figure
Strong Gravitational Lensing by Sgr A*
In recent years, there has been increasing recognition of the potential of
the galactic center as a probe of general relativity in the strong field. There
is almost certainly a black hole at Sgr A* in the galactic center, and this
would allow us the opportunity to probe dynamics near the exterior of the black
hole. In the last decade, there has been research into extreme gravitational
lensing in the galactic center. Unlike in most applications of gravitational
lensing, where the bending angle is of the order of several arc seconds, very
large bending angles are possible for light that closely approaches a black
hole. Photons may even loop multiple times around a black hole before reaching
the observer. There have been many proposals to use light's close approach to
the black hole as a probe of the black hole metric. Of particular interest is
the property of light lensed by the S stars orbiting in the galactic center.
This paper will review some of the attempts made to study extreme lensing as
well as extend the analysis of lensing by S stars. In particular, we are
interested in the effect of a Reissner-Nordstrom like 1/r^2 term in the metric
and how this would affect the properties of relativistic images.Comment: 13 pages, 9 figures. Submitted as invited review article for the GR19
issue of CQ
A Characteristic Planetary Feature in Double-Peaked, High-Magnification Microlensing Events
A significant fraction of microlensing planets have been discovered in
high-magnification events, and a significant fraction of these events exhibit a
double-peak structure at their peak. However, very wide or very close binaries
can also produce double-peaked high-magnification events, with the same gross
properties as those produced by planets. Traditionally, distinguishing between
these two interpretations has relied upon detailed modeling, which is both
time-consuming and generally does not provide insight into the observable
properties that allow discrimination between these two classes of models. We
study the morphologies of these two classes of double-peaked high-magnification
events, and identify a simple diagnostic that can be used to immediately
distinguish between perturbations caused by planetary and binary companions,
without detailed modeling. This diagnostic is based on the difference in the
shape of the intra-peak region of the light curves. The shape is smooth and
concave for binary lensing, while it tends to be either boxy or convex for
planetary lensing. In planetary lensing this intra-peak morphology is due to
the small, weak cusp of the planetary central caustic located between the two
stronger cusps. We apply this diagnostic to five observed double-peaked
high-magnification events to infer their underlying nature. A corollary of our
study is that good coverage of the intra-peak region of double-peaked
high-magnification events is likely to be important for their unique
interpretation.Comment: 6 pages, 3 figure
Estimating the parameters of the Sgr A* black hole
The measurement of relativistic effects around the galactic center may allow
in the near future to strongly constrain the parameters of the supermassive
black hole likely present at the galactic center (Sgr A*). As a by-product of
these measurements it would be possible to severely constrain, in addition,
also the parameters of the mass-density distributions of both the innermost
star cluster and the dark matter clump around the galactic center.Comment: Accepted for publication on General Relativity and Gravitation, 2010.
11 Pages, 1 Figur
Particle motion and gravitational lensing in the metric of a dilaton black hole in a de Sitter universe
We consider the metric exterior to a charged dilaton black hole in a de
Sitter universe. We study the motion of a test particle in this metric.
Conserved quantities are identified and the Hamilton-Jacobi method is employed
for the solutions of the equations of motion. At large distances from the black
hole the Hubble expansion of the universe modifies the effective potential such
that bound orbits could exist up to an upper limit of the angular momentum per
mass for the orbiting test particle. We then study the phenomenon of strong
field gravitational lensing by these black holes by extending the standard
formalism of strong lensing to the non-asymptotically flat dilaton-de Sitter
metric. Expressions for the various lensing quantities are obtained in terms of
the metric coefficients.Comment: 8 pages, RevTex, 1 eps figures; discussion improved; typos corrected;
references adde
Gravitational lensing by a charged black hole of string theory
We study gravitational lensing by the
Gibbons-Maeda-Garfinkle-Horowitz-Strominger (GMGHS) charged black hole of
heterotic string theory and obtain the angular position and magnification of
the relativistic images. Modeling the supermassive central object of the galaxy
as a GMGHS black hole, we estimate the numerical values of different
strong-lensing parameters. We find that there is no significant string effect
present in the lensing observables in the strong-gravity scenario.Comment: 6 page
Interacting Cosmic Fluids in Brans-Dicke Cosmology
We provide a detailed description for power-law scaling FRW cosmological
models in Brans-Dicke theory dominated by two interacting fluid components
during the expansion of the universe.Comment: 13 pages, 8 figure
Gravitational Lensing by Black Holes
We review the theoretical aspects of gravitational lensing by black holes,
and discuss the perspectives for realistic observations. We will first treat
lensing by spherically symmetric black holes, in which the formation of
infinite sequences of higher order images emerges in the clearest way. We will
then consider the effects of the spin of the black hole, with the formation of
giant higher order caustics and multiple images. Finally, we will consider the
perspectives for observations of black hole lensing, from the detection of
secondary images of stellar sources and spots on the accretion disk to the
interpretation of iron K-lines and direct imaging of the shadow of the black
hole.Comment: Invited article for the GRG special issue on lensing (P. Jetzer, Y.
Mellier and V. Perlick Eds.). 31 pages, 12 figure
Strong deflection limit of black hole gravitational lensing with arbitrary source distances
The gravitational field of supermassive black holes is able to strongly bend
light rays emitted by nearby sources. When the deflection angle exceeds ,
gravitational lensing can be analytically approximated by the so-called strong
deflection limit. In this paper we remove the conventional assumption of
sources very far from the black hole, considering the distance of the source as
an additional parameter in the lensing problem to be treated exactly. We find
expressions for critical curves, caustics and all lensing observables valid for
any position of the source up to the horizon. After analyzing the spherically
symmetric case we focus on the Kerr black hole, for which we present an
analytical 3-dimensional description of the higher order caustic tubes.Comment: 20 pages, 8 figures, appendix added. In press on Physical Review
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