1,364 research outputs found
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
Gravitational lensing in the strong field limit
We provide an analytic method to discriminate among different types of black
holes on the ground of their strong field gravitational lensing properties. We
expand the deflection angle of the photon in the neighbourhood of complete
capture, defining a strong field limit, in opposition to the standard weak
field limit. This expansion is worked out for a completely generic spherically
symmetric spacetime, without any reference to the field equations and just
assuming that the light ray follows the geodesics equation. We prove that the
deflection angle always diverges logarithmically when the minimum impact
parameter is reached. We apply this general formalism to Schwarzschild,
Reissner-Nordstrom and Janis-Newman-Winicour black holes. We then compare the
coefficients characterizing these metrics and find that different collapsed
objects are characterized by different strong field limits. The strong field
limit coefficients are directly connected to the observables, such as the
position and the magnification of the relativistic images. As a concrete
example, we consider the black hole at the centre of our galaxy and estimate
the optical resolution needed to investigate its strong field behaviour through
its relativistic images.Comment: 10 pages, 5 figures, in press on Physical Review
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
Electromagnetic waves around dilatonic stars and naked singularities
We study the propagation of classical electromagnetic waves on the simplest
four-dimensional spherically symmetric metric with a dilaton background field.
Solutions to the relevant equations are obtained perturbatively in a parameter
which measures the strength of the dilaton field (hence parameterizes the
departure from Schwarzschild geometry). The loss of energy from outgoing modes
is estimated as a back-scattering process against the dilaton background, which
would affect the luminosity of stars with a dilaton field. The radiation
emitted by a freely falling point-like source on such a background is also
studied by analytical and numerical methods.Comment: 9 pages, 1 figur
Trapped surfaces, horizons and exact solutions in higher dimensions
A very simple criterion to ascertain if (D-2)-surfaces are trapped in
arbitrary D-dimensional Lorentzian manifolds is given. The result is purely
geometric, independent of the particular gravitational theory, of any field
equations or of any other conditions. Many physical applications arise, a few
shown here: a definition of general horizon, which reduces to the standard one
in black holes/rings and other known cases; the classification of solutions
with a (D-2)-dimensional abelian group of motions and the invariance of the
trapping under simple dimensional reductions of the
Kaluza-Klein/string/M-theory type. Finally, a stronger result involving closed
trapped surfaces is presented. It provides in particular a simple sufficient
condition for their absence.Comment: 7 pages, no figures, final version to appear in Class. Quantum Gra
Microlensing toward crowded fields: Theory and applications to M31
We present a comprehensive treatment of the pixel-lensing theory and apply it
to lensing experiments and their results toward M31. Using distribution
functions for the distances, velocities, masses, and luminosities of stars, we
derive lensing event rates as a function of the event observables. In contrast
to the microlensing regime, in the pixel-lensing regime (crowded or unresolved
sources) the observables are the maximum excess flux of the source above a
background and the full width at half-maximum (FWHM) time of the event. To
calculate lensing event distribution functions depending on these observables
for the specific case of M31, we use data from the literature to construct a
model of M31, reproducing consistently photometry, kinematics and stellar
population. We predict the halo- and self-lensing event rates for bulge and
disk stars in M31 and treat events with and without finite source signatures
separately. We use the M31 photon noise profile and obtain the event rates as a
function of position, field of view, and S/N threshold at maximum
magnification. We calculate the expected rates for WeCAPP and for a potential
Advanced Camera for Surveys (ACS) lensing campaign. The detection of two events
with a peak signal-to-noise ratio larger than 10 and a timescale larger than 1
day in the WeCAPP 2000/2001 data is in good agreement with our theoretical
calculations. We investigate the luminosity function of lensed stars for noise
characteristics of WeCAPP and ACS. For the pixel-lensing regime, we derive the
probability distribution for the lens masses in M31 as a function of the FWHM
timescale, flux excess and color, including the errors of these observables.Comment: 45 pages, 27 figures LaTeX; corrected typos; published in the
Astrophysical Journal Supplemen
Penrose Limits, the Colliding Plane Wave Problem and the Classical String Backgrounds
We show how the Szekeres form of the line element is naturally adapted to
study Penrose limits in classical string backgrounds. Relating the "old"
colliding wave problem to the Penrose limiting procedure as employed in string
theory we discuss how two orthogonal Penrose limits uniquely determine the
underlying target space when certain symmetry is imposed. We construct a
conformally deformed background with two distinct, yet exactly solvable in
terms of the string theory on R-R backgrounds, Penrose limits. Exploiting
further the similarities between the two problems we find that the Penrose
limit of the gauged WZW Nappi-Witten universe is itself a gauged WZW plane wave
solution of Sfetsos and Tseytlin. Finally, we discuss some issues related to
singularity, show the existence of a large class of non-Hausdorff solutions
with Killing Cauchy Horizons and indicate a possible resolution of the problem
of the definition of quantum vacuum in string theory on these time-dependent
backgrounds.Comment: Some misprints corrected. Matches the version in print. To appear in
Classical & Quantum Gravit
Strong Gravitational Lensing in a Charged Squashed Kaluza- Klein Black hole
In this paper we investigate the strong gravitational lensing in a charged
squashed Kaluza-Klein black hole. We suppose that the supermassive black hole
in the galaxy center can be considered by a charged squashed Kaluza-Klein black
hole and then we study the strong gravitational lensing theory and estimate the
numerical values for parameters and observables of it. We explore the effects
of the scale of extra dimension and the charge of black hole
on these parameters and observables.Comment: 17 pages, 10 figure
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