4,279 research outputs found
Computational polarimetric microwave imaging
We propose a polarimetric microwave imaging technique that exploits recent
advances in computational imaging. We utilize a frequency-diverse cavity-backed
metasurface, allowing us to demonstrate high-resolution polarimetric imaging
using a single transceiver and frequency sweep over the operational microwave
bandwidth. The frequency-diverse metasurface imager greatly simplifies the
system architecture compared with active arrays and other conventional
microwave imaging approaches. We further develop the theoretical framework for
computational polarimetric imaging and validate the approach experimentally
using a multi-modal leaky cavity. The scalar approximation for the interaction
between the radiated waves and the target---often applied in microwave
computational imaging schemes---is thus extended to retrieve the susceptibility
tensors, and hence providing additional information about the targets.
Computational polarimetry has relevance for existing systems in the field that
extract polarimetric imagery, and particular for ground observation. A growing
number of short-range microwave imaging applications can also notably benefit
from computational polarimetry, particularly for imaging objects that are
difficult to reconstruct when assuming scalar estimations.Comment: 17 pages, 15 figure
Measurement of the earthshine polarization in the B, V, R, and I band as function of phase
The characterization of the polarimetric properties of the planet Earth is
important for the interpretation of expected observations and the planning of
future instruments. We present a multi-wavelengths and multi-phase set of
benchmark values for the polarization signal of the integrated light from the
planet Earth derived from new polarimetric observations of the earthshine
back-scattered from the Moon's dark side. Using a new, specially designed wide
field polarimeter we measured the fractional polarization of the earthshine in
the B, V, R and I filters for Earth phase angles alpha between 30{\deg} and
110{\deg}. The phase dependence of the earthshine polarization is fitted by a
function p x sin(alpha)^2. To determine the polarization of the planet Earth we
correct our earthshine measurements by a polarization efficiency function for
the lunar surface derived from measurements of lunar samples from the
literature. The polarization of the earthshine decreases towards longer
wavelengths and is about a factor 1.3 lower for the higher albedo highlands.
For mare regions the measured maximum polarization is about 13 % at quadrature
in the B band. The resulting fractional polarizations for Earth are 24.6 % for
the B band, 19.1 % for the V band, 13.5 % for the R band, and 8.3 % for the I
band. Together with literature values for the spectral reflectivity of Earth we
obtain a contrast between the polarized flux of the Earth and the (total) flux
of the Sun with an uncertainty of less than 20 % and we find that the best
phase to detect an Earth twin is around an Earth phase alpha=65{\deg}. The
polarimetric models of Earth-like planets from Stam (2008) are in qualitative
agreement with our results but there are also significant differences which
might guide more detailed computations.Comment: 14 pages, 14 figures, accepted for publication in Astronomy &
Astrophysic
SPICES: Spectro-Polarimetric Imaging and Characterization of Exoplanetary Systems
SPICES (Spectro-Polarimetric Imaging and Characterization of Exoplanetary
Systems) is a five-year M-class mission proposed to ESA Cosmic Vision. Its
purpose is to image and characterize long-period extrasolar planets and
circumstellar disks in the visible (450 - 900 nm) at a spectral resolution of
about 40 using both spectroscopy and polarimetry. By 2020/22, present and
near-term instruments will have found several tens of planets that SPICES will
be able to observe and study in detail. Equipped with a 1.5 m telescope, SPICES
can preferentially access exoplanets located at several AUs (0.5-10 AU) from
nearby stars (25 pc) with masses ranging from a few Jupiter masses to Super
Earths (2 Earth radii, 10 M) as well as circumstellar
disks as faint as a few times the zodiacal light in the Solar System
Detailed optical and near-infrared polarimetry, spectroscopy and broadband photometry of the afterglow of GRB 091018: Polarisation evolution
[Abridged] A number of phenomena have been observed in GRB afterglows that
defy explanation by simple versions of the standard fireball model, leading to
a variety of new models. Polarimetry can be a major independent diagnostic of
afterglow physics, probing the magnetic field properties and internal structure
of the GRB jets. In this paper we present the first high quality multi-night
polarimetric light curve of a Swift GRB afterglow, aimed at providing a well
calibrated dataset of a typical afterglow to serve as a benchmark system for
modelling afterglow polarisation behaviour. In particular, our dataset of the
afterglow of GRB 091018 (at redshift z=0.971) comprises optical linear
polarimetry (R band, 0.13 - 2.3 days after burst); circular polarimetry (R
band) and near-infrared linear polarimetry (Ks band). We add to that high
quality optical and near-infrared broadband light curves and spectral energy
distributions as well as afterglow spectroscopy. The linear polarisation varies
between 0 and 3%, with both long and short time scale variability visible. We
find an achromatic break in the afterglow light curve, which corresponds to
features in the polarimetric curve. We find that the data can be reproduced by
jet break models only if an additional polarised component of unknown nature is
present in the polarimetric curve. We probe the ordered magnetic field
component in the afterglow through our deep circular polarimetry, finding
P_circ < 0.15% (2 sigma), the deepest limit yet for a GRB afterglow, suggesting
ordered fields are weak, if at all present. Our simultaneous R and Ks band
polarimetry shows that dust induced polarisation in the host galaxy is likely
negligible.Comment: 20 pages, 14 figures, 3 tables. Accepted for publication in MNRAS.
Some figures are reduced in quality to comply with arXiv size requirement
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