571 research outputs found
Diffuse polarized emission associated with the Perseus cluster
We report on full-polarization radio observations of the Perseus cluster
(Abell 426) using the Westerbork Synthesis Radio Telescope (WSRT) at
wavelengths from 81-95 cm. We have employed a novel technique, Rotation Measure
synthesis (Brentjens and de Bruyn, 2005) to unravel the polarization properties
of the emission across the full field of view and detect polarized emission
over a wide range of RM from about 0 to 90 rad m^-2. The low RM emission is
associated with our Galaxy, while the high RM emission is associated with the
Perseus cluster. The latter reaches typical surface brightness levels of 0.5-1
mJy per beam and must be rather highly polarized. Most of the peripheral
polarized emission appears too bright, by about 1-2 orders of magnitude, to be
explainable as Thomson scattered emission of the central radio source off the
thermal electrons in the cluster. The bulk of the emission associated with the
Perseus cluster is probably related to buoyant bubbles of relativistic plasma,
probably relics from still active or now dormant AGN within the cluster. A
lenticular shaped structure measuring 0.5-1 Mpc is strikingly similar to the
structures predicted by Ensslin et al. (1998). At the western edge of the
cluster, we detect very long, linear structures that may be related to shocks
caused by infall of gas into the Perseus cluster.Comment: 18 pages, 17 figures, accepted by A&A, corrected small typo, added
referenc
Wide field polarimetry around the Perseus cluster at 350 MHz
This paper investigates the fascinating diffuse polarization structures at
350 MHz that have previously been tentatively attributed to the Perseus cluster
and, more specifically, tries to find out whether the structures are located at
(or near) the Perseus cluster, or in the Milky Way. A wide field, eight point
Westerbork Synthesis Radio Telescope mosaic of the area around the Perseus
cluster was observed in full polarization. The frequency range was 324 to 378
MHz and the resolution of the polarization maps was 2'x3'. The maps were
processed using Faraday rotation measure synthesis to counter bandwidth
depolarization. The RM-cube covers Faraday depths of -384 to +381 rad m^{-2} in
steps of 3 rad m^{-2}. There is emission all over the field at Faraday depths
between -50 and +100 rad m^{-2}. All previously observed structures were
detected. However, no compelling evidence was found supporting association of
those structures with either the Perseus cluster or large scale structure
formation gas flows in the Perseus-Pisces super cluster. On the contrary, one
of the structures is clearly associated with a Galactic depolarization canal at
1.41 GHz. Another large structure in polarized intensity, as well as Faraday
depth at a Faraday depth of +30 rad m^{-2}, coincides with a dark object in
WHAM H-alpha maps at a kinematic distance of 0.5 \pm 0.5 kpc. All diffuse
polarized emission at 350 MHz towards the Perseus cluster is most likely
located within 1 kpc from the Sun. The layers that emit the polarized radiation
are less than 40 pc/B|| thick.Comment: 16 pages, accepted for publication in A&
Faraday caustics: Singularities in the Faraday spectrum and their utility as probes of magnetic field properties
We describe singularities in the distribution of polarized intensity as a
function of Faraday depth (i.e. the Faraday spectrum) caused by line-of-sight
(LOS) magnetic field reversals. We call these features Faraday caustics because
of their similarity to optical caustics. They appear as sharply peaked and
asymmetric profiles in the Faraday spectrum, that have a tail that extends to
one side. The direction in which the tail extends depends on the way in which
the LOS magnetic field reversal occurs (either changing from oncoming to
retreating or vice versa). We describe how Faraday caustics will form
three-dimensional surfaces that relate to boundaries between regions where the
LOS magnetic field has opposite polarity. We present examples from simulations
of the predicted polarized synchrotron emission from the Milky Way. We derive
either the probability or luminosity distribution of Faraday caustics produced
in a Gaussian magnetic field distribution as a function of their strength, F,
and find that for strong Faraday caustics P(F)\proptoF^{-3} . If fully
resolved, this distribution is also shown to depend on the Taylor microscale,
which relates to the largest scale over which dissipation is important in a
turbulent flow.Comment: 14 pages, 9 figures, Accepted for publication in Astronomy &
Astrophysic
Wide-field LOFAR-LBA power-spectra analyses: Impact of calibration, polarization leakage and ionosphere
Contamination due to foregrounds (Galactic and Extra-galactic), calibration
errors and ionospheric effects pose major challenges in detection of the cosmic
21 cm signal in various Epoch of Reionization (EoR) experiments. We present the
results of a pilot study of a field centered on 3C196 using LOFAR Low Band
(56-70 MHz) observations, where we quantify various wide field and calibration
effects such as gain errors, polarized foregrounds, and ionospheric effects. We
observe a `pitchfork' structure in the 2D power spectrum of the polarized
intensity in delay-baseline space, which leaks into the modes beyond the
instrumental horizon (EoR/CD window). We show that this structure largely
arises due to strong instrumental polarization leakage () towards
{Cas\,A} ( kJy at 81 MHz, brightest source in northern sky), which is
far away from primary field of view. We measure an extremely small ionospheric
diffractive scale ( m at 60 MHz) towards {Cas\,A}
resembling pure Kolmogorov turbulence compared to
km towards zenith at 150 MHz for typical ionospheric conditions. This is one of
the smallest diffractive scales ever measured at these frequencies. Our work
provides insights in understanding the nature of aforementioned effects and
mitigating them in future Cosmic Dawn observations (e.g. with SKA-low and HERA)
in the same frequency window.Comment: 20 pages, 11 figures, accepted for publication in MNRA
Faraday Rotation Measure Synthesis
We extend the rotation measure work of Burn (1966) to the cases of limited
sampling of lambda squared space and non-constant emission spectra. We
introduce the rotation measure transfer function (RMTF), which is an excellent
predictor of n-pi ambiguity problems with the lambda squared coverage. Rotation
measure synthesis can be implemented very efficiently on modern computers.
Because the analysis is easily applied to wide fields, one can conduct very
fast RM surveys of weak spatially extended sources. Difficult situations, for
example multiple sources along the line of sight, are easily detected and
transparently handled. Under certain conditions, it is even possible to recover
the emission as a function of Faraday depth within a single cloud of ionized
gas. Rotation measure synthesis has already been successful in discovering
widespread, weak, polarized emission associated with the Perseus cluster (De
Bruyn and Brentjens, 2005). In simple, high signal to noise situations it is as
good as traditional linear fits to polarization angle versus lambda squared
plots. However, when the situation is more complex or very weak polarized
emission at high rotation measures is expected, it is the only viable option.Comment: 17 pages, 14 figures, accepted by A&A, added references, corrected
typo
Detection Thresholds and Bias Correction in Polarized Intensity
Detection thresholds in polarized intensity and polarization bias correction
are investigated for surveys where the polarization information is obtained
from RM synthesis. Considering unresolved sources with a single rotation
measure, a detection threshold of applied to the Faraday
spectrum will retrieve the RM with a false detection rate less than ,
but polarized intensity is more strongly biased than Ricean statistics suggest.
For a detection threshold of , the false detection rate
increases to ~4%, depending also on coverage and the extent of the
Faraday spectrum. Non-Gaussian noise in Stokes Q and U due to imperfect imaging
and calibration can be represented by a distribution that is the sum of a
Gaussian and an exponential. The non-Gaussian wings of the noise distribution
increase the false detection rate in polarized intensity by orders of
magnitude. Monte-Carlo simulations assuming non-Gaussian noise in Q and U, give
false detection rates at similar to Ricean false detection
rates at .Comment: 8 pages, 8 figures, accepted for publication in Publications of the
Astronomical Society of Australi
Faraday synthesis: The synergy of aperture and rotation measure synthesis
We introduce a new technique for imaging the polarized radio sky using
interferometric data. The new approach, which we call Faraday synthesis,
combines aperture and rotation measure synthesis imaging and deconvolution into
a single algorithm. This has several inherent advantages over the traditional
two-step technique, including improved sky plane resolution, fidelity, and
dynamic range. In addition, the direct visibility- to Faraday-space imaging
approach is a more sound foundation on which to build more sophisticated
deconvolution or inference algorithms. For testing purposes, we have
implemented a basic Faraday synthesis imaging software package including a
three-dimensional CLEAN deconvolution algorithm. We compare the results of this
new technique to those of the traditional approach using mock data. We find
many artifacts in the images made using the traditional approach that are not
present in the Faraday synthesis results. In all, we achieve a higher spatial
resolution, an improvement in dynamic range of about 20%, and a more accurate
reconstruction of low signal to noise source fluxes when using the Faraday
synthesis technique.Comment: 11 pages, 5 figures, submitted to A&
Foregrounds for observations of the cosmological 21 cm line: II. Westerbork observations of the fields around 3C196 and the North Celestial Pole
In the coming years a new insight into galaxy formation and the thermal
history of the Universe is expected to come from the detection of the highly
redshifted cosmological 21 cm line. The cosmological 21 cm line signal is
buried under Galactic and extragalactic foregrounds which are likely to be a
few orders of magnitude brighter. Strategies and techniques for effective
subtraction of these foreground sources require a detailed knowledge of their
structure in both intensity and polarization on the relevant angular scales of
1-30 arcmin. We present results from observations conducted with the Westerbork
telescope in the 140-160 MHz range with 2 arcmin resolution in two fields
located at intermediate Galactic latitude, centred around the bright quasar
3C196 and the North Celestial Pole. They were observed with the purpose of
characterizing the foreground properties in sky areas where actual observations
of the cosmological 21 cm line could be carried out. The polarization data were
analysed through the rotation measure synthesis technique. We have computed
total intensity and polarization angular power spectra. Total intensity maps
were carefully calibrated, reaching a high dynamic range, 150000:1 in the case
of the 3C196 field. [abridged]Comment: 20 pages, 22 figures, accepted for publication in A&A. A version with
full resolution figures is available at
http://www.astro.rug.nl/~bernardi/NCP_3C196/bernardi.pd
Galactic interstellar filaments as probed by LOFAR and Planck
Recent Low Frequency Array (LOFAR) observations at 115-175 MHz of a field at
medium Galactic latitudes (centered at the bright quasar 3C196) have shown
striking filamentary structures in polarization that extend over more than 4
degrees across the sky. In addition, the Planck satellite has released full sky
maps of the dust emission in polarization at 353GHz. The LOFAR data resolve
Faraday structures along the line of sight, whereas the Planck dust
polarization maps probe the orientation of the sky projected magnetic field
component. Hence, no apparent correlation between the two is expected. Here we
report a surprising, yet clear, correlation between the filamentary structures,
detected with LOFAR, and the magnetic field orientation, probed by the Planck
satellite. This finding points to a common, yet unclear, physical origin of the
two measurements in this specific area in the sky. A number of follow-up multi-
frequency studies are proposed to shed light on this unexpected finding.Comment: 6 pages, 4 figures, accepted for publication in MNRAS Letter
Broadband Meter-Wavelength Observations of Ionospheric Scintillation
Intensity scintillations of cosmic radio sources are used to study
astrophysical plasmas like the ionosphere, the solar wind, and the interstellar
medium. Normally these observations are relatively narrow band. With Low
Frequency Array (LOFAR) technology at the Kilpisj\"arvi Atmospheric Imaging
Receiver Array (KAIRA) station in northern Finland we have observed
scintillations over a 3 octave bandwidth. ``Parabolic arcs'', which were
discovered in interstellar scintillations of pulsars, can provide precise
estimates of the distance and velocity of the scattering plasma. Here we report
the first observations of such arcs in the ionosphere and the first broad-band
observations of arcs anywhere, raising hopes that study of the phenomenon may
similarly improve the analysis of ionospheric scintillations. These
observations were made of the strong natural radio source Cygnus-A and covered
the entire 30-250\,MHz band of KAIRA. Well-defined parabolic arcs were seen
early in the observations, before transit, and disappeared after transit
although scintillations continued to be obvious during the entire observation.
We show that this can be attributed to the structure of Cygnus-A. Initial
results from modeling these scintillation arcs are consistent with simultaneous
ionospheric soundings taken with other instruments, and indicate that
scattering is most likely to be associated more with the topside ionosphere
than the F-region peak altitude. Further modeling and possible extension to
interferometric observations, using international LOFAR stations, are
discussed.Comment: 11 pages, 17 figure
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