163 research outputs found
Parsec-scale structure in the warm ISM from polarized galactic radio background observations
We present multi-frequency polarization observations of the diffuse radio
synchrotron background modulated by Faraday rotation, in two directions of
positive latitude. No extended total intensity I is observed, which implies
that total intensity has no structure on scales smaller than approximately a
degree. Polarized intensity and polarization angle, however, show abundant
small-scale structure on scales from arcminutes to degrees. Rotation Measure
(RM) maps show coherent structure over many synthesized beams, but also abrupt
large changes over one beam. RM's from polarized extragalactic point sources
are correlated over the field in each of the two fields, indicating a galactic
component to the RM, but show no correlation with the RM map of the diffuse
radiation. The upper limit in structure in I puts constraints on the random and
regular components of the magnetic field in the galactic interstellar medium
and halo. The emission is partly depolarized so that the observed polarization
mostly originates from a nearby part of the medium. This explains the lack of
correlation between RM from diffuse emission and from extragalactic point
sources as the latter is built up over the entire path length through the
medium.Comment: To appear in "Astrophysical Polarized Backgrounds", Conference
Proceedings, eds S. Cecchini, S. Cortiglioni, R. Sault and C. Sbarra, in
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Structure in the local Galactic ISM on scales down to 1 pc, from multi-band radio polarization observations
We discuss observations of the linearly polarized component of the diffuse
galactic radio background. These observations, with an angular resolution of
4', were made with the Westerbork Synthesis Radio Telescope (WSRT) in 5
frequency bands in the range 341-375 MHz. The linearly polarized intensity P
(with polarized brightness temperature going up to 10K) shows a `cloudy'
structure, with characteristic scales of 15'-30', which contains relatively
long, but very narrow `canals' (essentially unresolved) in which P is only a
small fraction of that in the neighbouring beams.
These `canals' are generally seen in more than one frequency band, although
their appearance changes between bands. They are probably due to depolarization
within the synthesized beam, because the change in polarization angle across
the deepest `canals' is in general close to 90 degrees (or 270 etc.). These
very abrupt changes in polarization angle, which are seen only across the
`canals', seem to be accompanied by abrupt changes in the Rotation Measure
(RM), which may have the right magnitude to create the difference of close to
90 degrees in polarization angle, and thereby the `canals'.
The structure in the polarization maps is most likely due to Faraday rotation
modulation of the probably smooth polarized radiation emitted in the halo of
our Galaxy by the fairly local ISM (up to 500 pc). Therefore, the abrupt
changes of RM across the `canals' provide evidence for very thin (about 1 pc),
and relatively long transition regions in the ISM, across which the RM changes
by as much as 100%. Such drastic RM changes may well be due primarily to abrupt
changes in the magnetic field.Comment: 4 pages, 4 figures, to be published in A&A Letter
WSRT Faraday tomography of the Galactic ISM at \lambda \sim 0.86 m
We investigate the distribution and properties of Faraday rotating and
synchrotron emitting regions in the Galactic ISM in the direction of the
Galactic anti-centre. We apply Faraday tomography to a radio polarization
dataset that we obtained with the WSRT. We developed a new method to calculate
a linear fit to periodic data, which we use to determine rotation measures from
our polarization angle data. From simulations of a Faraday screen + noise we
could determine how compatible the data are with Faraday screens. An
unexpectedly large fraction of 14% of the lines-of-sight in our dataset show an
unresolved main component in the Faraday depth spectrum. For lines-of-sight
with a single unresolved component we demonstrate that a Faraday screen in
front of a synchrotron emitting region that contains a turbulent magnetic field
component can explain the data.Comment: 5 pages, 5 figures. Accepted for publication as a Letter to the
Editor in A&
The contribution of the Unresolved Extragalactic Radio Sources to the Brightness Temperature of the sky
The contribution of the Unresolved Extragalactic Radio Sources to the diffuse
brightness of the sky was evaluated using the source number - flux measurements
available in literature. We first optimized the fitting function of the data
based on number counts distribution. We then computed the brightness
temperature at various frequencies from 151 MHz to 8440 MHz and derived its
spectral dependence. As expected the frequency dependence can be described by a
power law with a spectral index , in agreement with the
flux emitted by the {\it steep spectrum} sources. The contribution of {\it flat
spectrum} sources becomes relevant at frequencies above several GHz. Using the
data available in literature we improved our knowledge of the brightness of the
unresolved extragalactic radio sources. The results obtained have general
validity and they can be used to disentangle the various contributions of the
sky brightness and to evaluate the CMB temperature.Comment: Accepted for publication in the Astrophysical Journa
Flow of Foams
We study the shear flow of two-dimensional foams, i.e., a monolayer of bubbles floating on a soapy solution. We successfully connect local and global flow behaviourStichting FOMUBL - phd migration 201
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