235 research outputs found

### Mid-infrared microlensing of accretion disc and dusty torus in quasars: effects on flux ratio anomalies

Multiply-imaged quasars and AGNs observed in the mid-infrared (MIR) range are
commonly assumed to be unaffected by the microlensing produced by the stars in
their lensing galaxy. In this paper, we investigate the validity domain of this
assumption. Indeed, that premise disregards microlensing of the accretion disc
in the MIR range, and does not account for recent progress in our knowledge of
the dusty torus. To simulate microlensing, we first built a simplified image of
the quasar composed of an accretion disc, and of a larger ring-like torus. The
mock quasars are then microlensed using an inverse ray-shooting code. We
simulated the wavelength and size dependence of microlensing for different
lensed image types and fraction of compact objects projected in the lens. This
allows us to derive magnification probabilities as a function of wavelength, as
well as to calculate the microlensing-induced deformation of the spectral
energy distribution of the lensed images. We find that microlensing variations
as large as 0.1 mag are very common at 11 microns (observer-frame). The main
signal comes from microlensing of the accretion disc, which may be significant
even when the fraction of flux from the disc is as small as 5 % of the total
flux. We also show that the torus of sources with Lbol <~ 10^45 erg/s is
expected to be noticeably microlensed. Microlensing may thus be used to get
insight into the rest near-infrared inner structure of AGNs. Finally, we
investigate whether microlensing in the mid-infrared can alter the so-called
Rcusp relation that links the fluxes of the lensed images triplet produced when
the source lies close to a cusp macro-caustic. This relation is commonly used
to identify massive (dark-matter) substructures in lensing galaxies. We find
that significant deviations from Rcusp may be expected, which means that
microlensing can explain part of the flux ratio problem.Comment: Updated to match the version published in Astronomy and Astrophysics.
12 pages. Abridged version of the abstract. Microlensing maps and source
profiles used in the simulations are available via CDS -
http://vizier.cfa.harvard.edu/viz-bin/VizieR?-source=J/A+A/553/A5

### Cosmology with gravitationally lensed repeating Fast Radio Bursts

High-precision cosmological probes have revealed a small but significant
tension between the parameters measured with different techniques, among which
there is one based on time delays in gravitational lenses. We discuss a new way
of using time delays for cosmology, taking advantage of the extreme precision
expected for lensed fast radio bursts (FRBs), which are short flashes of radio
emission originating at cosmological distances. With coherent methods, the
achievable precision is sufficient for measuring how time delays change over
the months and years, which can also be interpreted as differential redshifts
between the images. It turns out that uncertainties arising from the unknown
mass distribution of gravitational lenses can be eliminated by combining time
delays with their time derivatives. Other effects, most importantly relative
proper motions, can be measured accurately and disentangled from the
cosmological effects. With a mock sample of simulated lenses, we show that it
may be possible to attain strong constraints on cosmological parameters.
Finally, the lensed images can be used as galactic interferometer to resolve
structures and motions of the burst sources with incredibly high resolution and
help reveal their physical nature, which is currently unknown.Comment: minor revision, published in A&A, 15 page

### Models for the lens and source of B0218+357 - A LensClean approach to determine H0

B0218+357 is one of the most promising systems to determine the Hubble
constant from gravitational lenses. Consisting of two bright resolved images
plus an Einstein ring, it provides better constraints for the mass model than
other systems. The main problem left until now was the poorly determined
position of the lensing galaxy.
After presenting results from classical lens modelling, we apply our improved
version of LensClean which utilizes the Einstein ring for lens modelling
purposes. The primary result using isothermal models is a well defined lens
position which allows the first reliable measurement of the Hubble constant
from this system. The result of H0=(78+-6) km/s/Mpc (2 sigma) is high compared
with other lenses but compatible with the HST key project and WMAP results. We
furthermore discuss effects of different radial mass profiles. The power-law
exponent of the potential is constrained by VLBI data to be beta=1.04+-0.02,
very close to isothermal. The effect on H0 is expected to be very small.
We also present a composite map (lensed and unlensed) which shows the rich
structure of B0218+357 on scales from milli-arcseconds to arcseconds. Finally
we use a comparison of observations at different frequencies to investigate the
question of possible weakening of one of the images by propagation effects
and/or source shifts with frequency. The data clearly favour the model of
significant extinction without noticeable source position shifts.
The technical details of our variant of the LensClean method are presented in
the accompanying Paper I.Comment: To appear in MNRAS together with Paper I (astro-ph/0312262). 18
Pages. Full resolution version available at
http://www.astro.physik.uni-potsdam.de/~olaf or on reques

### Detection of microgauss coherent magnetic fields in a galaxy five billion years ago

Magnetic fields play a pivotal role in the physics of interstellar medium in
galaxies, but there are few observational constraints on how they evolve across
cosmic time. Spatially resolved synchrotron polarization maps at radio
wavelengths reveal well-ordered large-scale magnetic fields in nearby galaxies
that are believed to grow from a seed field via a dynamo effect. To directly
test and characterize this theory requires magnetic field strength and geometry
measurements in cosmologically distant galaxies, which are challenging to
obtain due to the limited sensitivity and angular resolution of current radio
telescopes. Here, we report the cleanest measurements yet of magnetic fields in
a galaxy beyond the local volume, free of the systematics traditional
techniques would encounter. By exploiting the scenario where the polarized
radio emission from a background source is gravitationally lensed by a
foreground galaxy at z = 0.439 using broadband radio polarization data, we
detected coherent $\mu$G magnetic fields in the lensing disk galaxy as seen 4.6
Gyrs ago, with similar strength and geometry to local volume galaxies. This is
the highest redshift galaxy whose observed coherent magnetic field property is
compatible with a mean-field dynamo origin.Comment: 29 pages, 5 figures (including Supplementary Information). Published
in Nature Astronomy on August 28, 201

### From planes to spheres: About gravitational lens magnifications

We discuss the classic theorem according to which a gravitational lens always
produces a total magnification greater than unity. This theorem seems to
contradict the conservation of total flux from a lensed source. The standard
solution to this paradox is based on the exact definition of the reference
'unlensed' situation.
We calculate magnifications and amplifications for general lensing scenarios
not limited to regions close to the optical axis. In this way the formalism is
naturally extended from tangential planes for the source and lensed images to
complete spheres. We derive the lensing potential theory on the sphere and find
that the Poisson equation is modified by an additional source term that is
related to the mean density and to the Newtonian potential at the positions of
observer and source. This new term generally reduces the magnification, to
below unity far from the optical axis, and ensures conservation of the total
photon number received on a sphere around the source.
This discussion does not affect the validity of the 'focusing theorem', in
which the unlensed situation is defined to have an unchanged affine distance
between source and observer. The focusing theorem does not contradict flux
conservation, because the mean total magnification directly corresponds to
different areas of the source sphere in the lensed and unlensed situation. We
argue that a constant affine distance does not define an astronomically
meaningful reference.
By exchanging source and observer, we confirm that magnification and
amplification differ according to Etherington's reciprocity law, so that
surface brightness is no longer strictly conserved. [ abridged ]Comment: MNRAS accepted. 15 pages, 6 figure

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