140 research outputs found
The DRAO 26-m Large Scale Polarization Survey at 1.41 GHz
The Effelsberg telescope as well as the DRAO synthesis telescope are
currently surveying the Galactic polarized emission at 21 cm in detail. These
new surveys reveal an unexpected richness of small-scale structures in the
polarized sky. However, observations made with synthesis or single-dish
telescopes are not on absolute intensity scales and therefore lack information
about the large-scale distribution of polarized emission to a different degree.
Until now, absolutely calibrated polarization data from the Leiden/Dwingeloo
polarization surveys are used to recover the missing spatial information.
However, these surveys cannot meet the requirements of the recent survey
projects regarding sampling and noise and new polarization observation were
initiated to complement the Leiden/Dwingeloo Survey. In this paper we will
outline the observation and report on the progress for a new polarization
survey of the northern sky with the 26-m telescope of the DRAO.Comment: 5 pages, 6 figure
Rotation Measure Synthesis of Galactic Polarized Emission with the DRAO 26-m Telescope
Radio polarimetry at decimetre wavelengths is the principal source of
information on the Galactic magnetic field. The diffuse polarized emission is
strongly influenced by Faraday rotation in the magneto-ionic medium and
rotation measure is the prime quantity of interest, implying that all Stokes
parameters must be measured over wide frequency bands with many frequency
channels. The DRAO 26-m Telescope has been equipped with a wideband feed, a
polarization transducer to deliver both hands of circular polarization, and a
receiver, all operating from 1277 to 1762 MHz. Half-power beamwidth is between
40 and 30 arcminutes. A digital FPGA spectrometer, based on commercially
available components, produces all Stokes parameters in 2048 frequency channels
over a 485-MHz bandwidth. Signals are digitized to 8 bits and a Fast Fourier
Transform is applied to each data stream. Stokes parameters are then generated
in each frequency channel. This instrument is in use at DRAO for a Northern sky
polarization survey. Observations consist of scans up and down the Meridian at
a drive rate of 0.9 degree per minute to give complete coverage of the sky
between declinations -30 degree and 90 degree. This paper presents a complete
description of the receiver and data acquisition system. Only a small fraction
of the frequency band of operation is allocated for radio astronomy, and about
20 percent of the data are lost to interference. The first 8 percent of data
from the survey are used for a proof-of-concept study, which has led to the
first application of Rotation Measure Synthesis to the diffuse Galactic
emission obtained with a single-antenna telescope. We find rotation measure
values for the diffuse emission as high as approximately 100 rad per square
metre, much higher than recorded in earlier work.Comment: Accepted for publication in The Astronomical Journa
The local Galactic magnetic field in the direction of Geminga
The Milagro hot spot A, close to the Galactic anticenter direction, has been
tentatively attributed to cosmic rays from a local reservoir (at a distance
~100 pc), freely streaming along diverging and smooth magnetic field lines.
This is at variance with the geometry of the ~kpc scale Galactic magnetic
field, which is known to be aligned with the spiral arms. We investigate the
information available on the geometry of the magnetic field on the scales (~100
pc) of relevance here. The magnetic field immediately upstream of the
heliosphere has been investigated by previous authors by modeling the
interaction of this field with the solar wind. At larger distances, we use the
dispersion measure and the rotation measure of nearby pulsars (especially
towards the third Galactic quadrant). Additional information about the local
field towards the North Polar Spur is taken from previous studies of the
diffuse radio emission and the polarization of starlight. The asymmetry of the
heliosphere with respect to the incoming interstellar medium implies a magnetic
field almost orthogonal to the local spiral arm, in the general direction of
hot spot A, but more to the south. This is in good agreement with the nearby
pulsar data on the one side, and the North Polar Spur data on the other. The
local magnetic field on scales of ~100 parsecs around the Sun seems to be
oriented so as to provide a direct connection between the Solar system and a
possible site of the Geminga supernova; the residual angular difference and the
shape and orientation of the Milagro hot spot can be attributed to the field
trailing in the wake of the heliosphere.Comment: 5 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
The magnetic field of the Large Magellanic Cloud revealed through Faraday rotation
We have measured the Faraday rotation toward a large sample of polarized
radio sources behind the Large Magellanic Cloud (LMC), to determine the
structure of this galaxy's magnetic field. The magnetic field of the LMC
consists of a coherent axisymmetric spiral of field strength ~1 microgauss.
Strong fluctuations in the magnetic field are also seen, on small (<0.5
parsecs) and large (~100 parsecs) scales. The significant bursts of recent star
formation and supernova activity in the LMC argue against standard dynamo
theory, adding to the growing evidence for rapid field amplification in
galaxies.Comment: 15 pages, including 3 embedded EPS figures (1 color, 2 b/w) plus
supporting on-line material; uses scicite.sty. To appear in Science, vol 307,
number 5715 (11 March 2005
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
Polarization surveys of the Galaxy
We report on sensitive 21cm and 11cm polarization surveys of the Galactic
plane carried out with the Effelsberg 100-m telescope at arcmin angular
resolution and some related work. Highly structured polarized emission is seen
along the Galactic plane as well as up to very high Galactic latitudes. These
observations reflect Faraday effects in the interstellar medium. Polarized
foreground and background components along the line of sight, modified by
Faraday rotation and depolarization, add in a complex way. The amplitudes of
polarized emission features are highly frequency dependent. Small-scale
components decrease in amplitude rapidly with increasing frequency. We stress
the need for sensitive absolutely calibrated polarization data. These are
essential for baseline setting and a correct interpretation of small-scale
structures. Absolutely calibrated data are also needed to estimate the
high-frequency polarized background. A recent study of polarized emission
observed across the local Taurus-Auriga molecular cloud complexes indicates
excessive synchrotron emission within a few hundred parsecs. These results
suggest that possibly a large fraction of the Galactic high latitude total
intensity and polarized emission is of local origin.Comment: 6 pages with 2 PS figures. To be published in "Astrophysical
Polarized Backgrounds", eds. S. Cecchini, S. Cortiglioni, R. Sault and C.
Sbarra, AIP Conf. Pro
The Global Magneto-Ionic Medium Survey: A Faraday Depth Survey of the Northern Sky Covering 1280–1750 MHz
The Galactic interstellar medium hosts a significant magnetic field, which can be probed through the synchrotron
emission produced from its interaction with relativistic electrons. Linearly polarized synchrotron emission is
generated throughout the Galaxy and, at longer wavelengths, modified along nearly every path by Faraday
rotation in the intervening magneto-ionic medium. Full characterization of the polarized emission requires
wideband observations with many frequency channels. We have surveyed polarized radio emission from the
Northern sky over the range 1280–1750 MHz, with channel width 236.8 kHz, using the John A. Galt Telescope
(diameter 25.6 m) at the Dominion Radio Astrophysical Observatory, as part of the Global Magneto-Ionic
Medium Survey. The survey covered 72% of the sky, decl. −30° to +87° at all R.A. The intensity scale was
absolutely calibrated, based on the flux density and spectral index of Cygnus A. Polarization angle was
calibrated using the extended polarized emission of the Fan Region. Data are presented as brightness
temperatures with angular resolution 40′. Sensitivity in Stokes Q and U is 45 mK rms in a 1.18 MHz band. We
have applied rotation measure synthesis to the data to obtain a Faraday depth cube of resolution 150 rad m−2 and
sensitivity 3 mK rms of polarized intensity. Features in Faraday depth up to a width of 110 rad m−2 are
represented. The maximum detectable Faraday depth is ±2 × 104 rad m−2
. The survey data are available at the
Canadian Astronomy Data Centre
Faraday Tomography of the North Polar Spur: Constraints on the distance to the Spur and on the Magnetic Field of the Galaxy
We present radio continuum and polarization images of the North Polar Spur
(NPS) from the Global Magneto-Ionic Medium Survey (GMIMS) conducted with the
Dominion Radio Astrophysical Observatory 26-m Telescope. We fit polarization
angle versus wavelength squared over 2048 frequency channels from 1280 to 1750
MHz to obtain a Faraday Rotation Measure (RM) map of the NPS. Combining this RM
map with a published Faraday depth map of the entire Galaxy in this direction,
we derive the Faraday depth introduced by the NPS and the Galactic interstellar
medium (ISM) in front of and behind the NPS. The Faraday depth contributed by
the NPS is close to zero, indicating that the NPS is an emitting only feature.
The Faraday depth caused by the ISM in front of the NPS is consistent with zero
at b>50 degree, implying that this part of the NPS is local at a distance of
approximately several hundred parsecs. The Faraday depth contributed by the ISM
behind the NPS gradually increases with Galactic latitude up to b=44 degree,
and decreases at higher Galactic latitudes. This implies that either the part
of the NPS at b<44 degree is distant or the NPS is local but there is a sign
change of the large-scale magnetic field. If the NPS is local, there is then no
evidence for a large-scale anti-symmetry pattern in the Faraday depth of the
Milky Way. The Faraday depth introduced by the ISM behind the NPS at latitudes
b>50 degree can be explained by including a coherent vertical magnetic field.Comment: 9 pages, 8 figures, accepted for publication in ApJ. Some figures
have been degraded to reduce sizes, for a high resolution version, see
http://physics.usyd.edu.au/~xhsun/ms_nps.pd
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