89 research outputs found
A Limit on the Polarized Anisotropy of the Cosmic Microwave Background at Subdegree Angular Scales
A ground-based polarimeter, PIQUE, operating at 90 GHz has set a new limit on
the magnitude of any polarized anisotropy in the cosmic microwave background.
The combination of the scan strategy and full width half maximum beam of 0.235
degrees gives broad window functions with average multipoles, l = 211+294-146
and l = 212+229-135 for the E- and B-mode window functions, respectively. A
joint likelihood analysis yields simultaneous 95% confidence level flat band
power limits of 14 and 13 microkelvin on the amplitudes of the E- and B-mode
angular power spectra, respectively. Assuming no B-modes, a 95% confidence
limit of 10 microkelvin is placed on the amplitude of the E-mode angular power
spectrum alone.Comment: 4 pages, 3 figures, submitted to Astrophysical Journal Letter
A Highly Ordered Faraday-Rotation Structure in the Interstellar Medium
We describe a Faraday-rotation structure in the Interstellar Medium detected
through polarimetric imaging at 1420 MHz from the Canadian Galactic Plane
Survey (CGPS). The structure, at l=91.8, b=-2.5, has an extent of ~2 degree,
within which polarization angle varies smoothly over a range of ~100 degree.
Polarized intensity also varies smoothly, showing a central peak within an
outer shell. This region is in sharp contrast to its surroundings, where
low-level chaotic polarization structure occurs on arcminute scales. The
Faraday-rotation structure has no counterpart in radio total intensity, and is
unrelated to known objects along the line of sight, which include a Lynds
Bright Nebula, LBN 416, and the star cluster M39 (NGC7092). It is interpreted
as a smooth enhancement of electron density. The absence of a counterpart,
either in optical emission or in total intensity, establishes a lower limit to
its distance. An upper limit is determined by the strong beam depolarization in
this direction. At a probable distance of 350 +/- 50 pc, the size of the object
is 10 pc, the enhancement of electron density is 1.7 cm-3, and the mass of
ionized gas is 23 M_sun. It has a very smooth internal magnetic field of
strength 3 microG, slightly enhanced above the ambient field. G91.8-2.5 is the
second such object to be discovered in the CGPS, and it seems likely that such
structures are common in the Magneto-Ionic Medium.Comment: 16 pages, 5 figures, ApJ accepte
Wilkinson Microwave Anisotropy Probe (WMAP) First Year Observations: TE Polarization
The Wilkinson Microwave Anisotropy Probe (WMAP) has mapped the full sky in
Stokes I, Q, and U parameters at frequencies 23, 33, 41, 61, and 94 GHz. We
detect correlations between the temperature and polarization maps significant
at more than 10 standard deviations. The correlations are present in all WMAP
frequency bands with similar amplitude from 23 to 94 GHz, and are consistent
with a superposition of a CMB signal with a weak foreground. The fitted CMB
component is robust against different data combinations and fitting techniques.
On small angular scales theta < 5 deg, the WMAP data show the
temperature-polarization correlation expected from adiabatic perturbations in
the temperature power spectrum. The data for l > 20 agree well with the signal
predicted solely from the temperature power spectra, with no additional free
parameters. We detect excess power on large angular scales (theta > 10 deg)
compared to predictions based on the temperature power spectra alone. The
excess power is well described by reionization at redshift 11 < z_r < 30 at 95%
confidence, depending on the ionization history. A model-independent fit to
reionization optical depth yields results consistent with the best-fit
LambdaCDM model, with best fit value tau = 0.17 +- 0.04 at 68% confidence,
including systematic and foreground uncertainties. This value is larger than
expected given the detection of a Gunn-Peterson trough in the absorption
spectra of distant quasars, and implies that the universe has a complex
ionization history: WMAP has detected the signal from an early epoch of
reionization.Comment: Replaced with version accepted by ApJ; Fig. 9 caption fixed. One of
13 companion papers on first-year WMAP results; 33 pages, 9 figures; version
with higher quality figures is at http://lambda.gsfc.nasa.gov
TRIS I: Absolute Measurements of the Sky Brightness Temperature at 0.6, 0.82 and 2.5 GHz
At frequencies close to 1 GHz the sky diffuse radiation is a superposition of
radiation of Galactic origin, the 3 K Relic or Cosmic Microwave Background
Radiation, and the signal produced by unresolved extragalactic sources. Because
of their different origin and space distribution the relative importance of the
three components varies with frequency and depends on the direction of
observation. With the aim of disentangling the components we built TRIS, a
system of three radiometers, and studied the temperature of the sky at , and GHz using geometrically scaled antennas
with identical beams (HPBW = ). Observations
included drift scans along a circle at constant declination
which provided the dependence of the sky signal on the
Right Ascension, and absolute measurement of the sky temperature at selected
points along the same scan circle. TRIS was installed at Campo Imperatore (lat.
= N, long.= , elevation = 2000 m a.s.l.) in
Central Italy, close to the Gran Sasso Laboratory.Comment: Accepted for publication in The Astrophysical Journa
TRIS III: the diffuse galactic radio emission at
We present values of temperature and spectral index of the galactic diffuse
radiation measured at 600 and 820 MHz along a 24 hours right ascension circle
at declination . They have been obtained from a subset of
absolute measurements of the sky temperature made with TRIS, an experiment
devoted to the measurement of the Cosmic Microwave Background temperature at
decimetric-wavelengths with an angular resolution of about .
Our analysis confirms the preexisting picture of the galactic diffuse
emission at decimetric wavelength and improves the accuracy of the measurable
quantities. In particular, the signal coming from the halo has a spectral index
in the range above 600 MHz, depending on the sky position. In the
disk, at TRIS angular resolution, the free-free emission accounts for the 11%
of the overall signal at 600 MHz and 21% at 1420 MHz. The polarized component
of the galactic emission, evaluated from the survey by Brouw and Spoelstra,
affects the observations at TRIS angular resolution by less than 3% at 820 MHz
and less than 2% at 600 MHz. Within the uncertainties, our determination of the
galactic spectral index is practically unaffected by the correction for
polarization.
Since the overall error budget of the sky temperatures measured by TRIS at
600 MHz, that is 66 mK(systematic)18 mK (statistical), is definitely smaller
than those reported in previous measurements at the same frequency, our data
have been used to discuss the zero levels of the sky maps at 150, 408, 820 and
1420 MHz in literature. Concerning the 408 MHz survey, limiting our attention
to the patch of sky corresponding to the region observed by TRIS, we suggest a
correction of the base-level of K.Comment: Accepted for publication in the Astrophysical Journa
A New approach for a Galactic Synchrotron Polarized Emission Template in the Microwave Range
We present a new approach in modelling the polarized Galactic synchrotron
emission in the microwave range (20-100 GHz), where this radiation is expected
to play the leading role in contaminating the Cosmic Microwave Background (CMB)
data. Our method is based on real surveys and aims at providing the real
spatial distributions of both polarized intensity and polarization angles. Its
main features are the modelling of a polarization horizon to determine the
polarized intensity and the use of starlight optical data to model the
polarization angle pattern. Our results are consistent with several existing
data, and our template is virtually free from Faraday rotation effects as
required at frequencies in the cosmological window.Comment: 12 pages with 15 figures, submitted to MNRA
Radio polarimetric imaging of the interstellar medium: magnetic field and diffuse ionized gas structure near the W3/W4/W5/HB3 complex
We have used polarimetric imaging to study the magneto-ionic medium of the
Galaxy, obtaining 1420 MHz images with an angular resolution of 1' over more
than 40 square-degrees of sky around the W3/W4/W5/HB3 HII region/SNR complex in
the Perseus Arm. Features detected in polarization angle are imposed on the
linearly polarized Galactic synchrotron background emission by Faraday rotation
arising in foreground ionized gas having an emission measure as low as 1
cm^{-6} pc. Several new remarkable phenomena have been identified, including:
mottled polarization arising from random fluctuations in a magneto-ionic screen
that we identify with a medium in the Perseus Arm, probably in the vicinity of
the HII regions themselves; depolarization arising from very high rotation
measures (several times 10^3 rad m^{-2}) and rotation measure gradients due to
the dense, turbulent environs of the HII regions; highly ordered features
spanning up to several degrees; and an extended influence of the HII regions
beyond the boundaries defined by earlier observations. In particular, the
effects of an extended, low-density ionized halo around the HII region W4 are
evident, probably an example of the extended HII envelopes postulated as the
origin of weak recombination-line emission detected from the Galactic ridge.
Our polarization observations can be understood if the uniform magnetic field
component in this envelope scales with the square-root of electron density and
is 20 microG at the edge of the depolarized region around W4, although this is
probably an over-estimate since the random field component will have a
significant effect.Comment: 18 pages, 8 figures (7 jpeg and 1 postscript), accepted for
publication in the Astrophysical Journa
GMIMS: The Global Magneto-Ionic Medium Survey
The Global Magneto-Ionic Medium Survey (GMIMS) is a project to map the
diffuse polarized emission over the entire sky, Northern and Southern
hemispheres, from 300 MHz to 1.8 GHz. With an angular resolution of 30 - 60
arcmin and a frequency resolution of 1 MHz or better, GMIMS will provide the
first spectro-polarimetric data set of the large-scale polarized emission over
the entire sky, observed with single-dish telescopes. GMIMS will provide an
invaluable resource for studies of the magneto-ionic medium of the Galaxy in
the local disk, halo, and its transition.Comment: To appear in Cosmic Magnetic Fields: From Planets, to Stars and
Galaxies, eds. K.G. Strassmeier, A.G. Kosovichev & J.E. Beckma
Polarization angular spectra of Galactic synchrotron emission on arcminute scales
We study the angular power spectra of the polarized component of the Galactic
synchrotron emission in the 28-deg^2 Test Region of the Southern Galactic Plane
Survey at 1.4 GHz. These data were obtained by the Australia Telescope Compact
Array and allow us to investigate angular power spectra down to arcminute
scales. We find that, at this frequency, the polarization spectra for E- and
B-modes seem to be affected by Faraday rotation produced in compact foreground
screens. A different behavior is shown by the angular spectrum of the polarized
intensity PI=\sqrt{Q^2+U^2}. This is well fitted by a power law with slope
\~1.7, which agrees with higher frequency results and can probably be more
confidently extrapolated to the cosmological window.Comment: 11 pages, 6 figures, accepted for publication in Ap
A very brief description of LOFAR - the Low Frequency Array
LOFAR (Low Frequency Array) is an innovative radio telescope optimized for
the frequency range 30-240 MHz. The telescope is realized as a phased aperture
array without any moving parts. Digital beam forming allows the telescope to
point to any part of the sky within a second. Transient buffering makes
retrospective imaging of explosive short-term events possible. The scientific
focus of LOFAR will initially be on four key science projects (KSPs): 1)
detection of the formation of the very first stars and galaxies in the universe
during the so-called epoch of reionization by measuring the power spectrum of
the neutral hydrogen 21-cm line (Shaver et al. 1999) on the ~5' scale; 2)
low-frequency surveys of the sky with of order expected new sources; 3)
all-sky monitoring and detection of transient radio sources such as gamma-ray
bursts, x-ray binaries, and exo-planets (Farrell et al. 2004); and 4) radio
detection of ultra-high energy cosmic rays and neutrinos (Falcke & Gorham 2003)
allowing for the first time access to particles beyond 10^21 eV (Scholten et
al. 2006). Apart from the KSPs open access for smaller projects is also
planned. Here we give a brief description of the telescope.Comment: 2 pages, IAU GA 2006, Highlights of Astronomy, Volume 14, K.A. van
der Hucht, e
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