1,383 research outputs found
A Multifrequency Analysis of the Polarized Diffuse Galactic Radio Emission at Degree Scales
The polarized diffuse Galactic radio emission, mainly synchrotron emission,
is expected to be one of the most relevant source of astrophysical
contamination at low and moderate multipoles in cosmic microwave background
polarization anisotropy experiments at frequencies lower then 50 to 100 GHz. We
present here preliminary results based on a recent analysis of the Leiden
surveys covering about 50% of the sky at low as well as at middle and high
Galactic latitudes. By implementing specific interpolation methods to deal with
these data, which show a large variation of the sampling across the sky, we
produce maps of the polarized diffuse Galactic synchrotron component at
frequencies between 408 and 1411 MHz with pixel sizes larger or equal to about
0.92 degrees. We derive the angular power spectrum of this component for the
whole covered region and for three patches in the sky significantly oversampled
with respect to the average and at different Galactic latitudes. We find
multipole spectral indices typically ranging between about -3 and about -1,
according to the considered frequency and sky region. At frequencies higher or
equal to 610 MHz, the frequency spectral indices observed in the considered sky
regions are about -3.5, compatible with an intrinsic frequency spectral index
of about -5.8 and a depolarization due to Faraday rotation with a rotation
measure RM of about 15 radians per square meter. This implies that the observed
angular power spectrum of the polarized signal is about 85% or 20% of the
intrinsic one at 1411 MHz or 820 MHz respectively.Comment: 6 pages, 5 figures. to appear in S.Cecchini et al., Astrophysical
Polarized Backgrounds, AIP Conf. Proceeding
Impact of foregrounds on Cosmic Microwave Background maps
We discuss the possible impact of astrophysical foregrounds on three recent
exciting results of Cosmic Microwave Background (CMB) experiments: the WMAP
measurements of the temperature-polarization (TE) correlation power spectrum,
the detection of CMB polarization fluctuations on degree scales by the DASI
experiment, and the excess power on arcminute scales reported by the CBI and
BIMA groups. A big contribution from the Galactic synchrotron emission to the
TE power spectrum on large angular scales is indeed expected, in the lower
frequency WMAP channels, based on current, albeit very uncertain, models; at
higher frequencies the rapid decrease of the synchrotron signal may be, to some
extent, compensated by polarized dust emission. Recent measurements of
polarization properties of extragalactic radio sources at high radio frequency
indicate that their contamination of the CMB polarization on degree scales at
30 GHz is substantially below the expected CMB E-mode amplitude. Adding the
synchrotron contribution, we estimate that the overall foreground contamination
of the signal detected by DASI may be significant but not dominant. The excess
power on arc-min scales detected by the BIMA experiment may be due to
galactic-scale Sunyaev-Zeldovich effects, if the proto-galactic gas is heated
to its virial temperature and its cooling time is comparable to the Hubble time
at the epoch of galaxy formation. A substantial contamination by radio sources
of the signal reported by the CBI group on scales somewhat larger than BIMA's
cannot be easily ruled out.Comment: 10 pages, 5 figures, to appear in proc. int. conf. "Thinking,
Observing and Mining the Universe", Sorrento, Sept. 200
Data Streams from the Low Frequency Instrument On-Board the Planck Satellite: Statistical Analysis and Compression Efficiency
The expected data rate produced by the Low Frequency Instrument (LFI) planned
to fly on the ESA Planck mission in 2007, is over a factor 8 larger than the
bandwidth allowed by the spacecraft transmission system to download the LFI
data. We discuss the application of lossless compression to Planck/LFI data
streams in order to reduce the overall data flow. We perform both theoretical
analysis and experimental tests using realistically simulated data streams in
order to fix the statistical properties of the signal and the maximal
compression rate allowed by several lossless compression algorithms. We studied
the influence of signal composition and of acquisition parameters on the
compression rate Cr and develop a semiempirical formalism to account for it.
The best performing compressor tested up to now is the arithmetic compression
of order 1, designed for optimizing the compression of white noise like
signals, which allows an overall compression rate = 2.65 +/- 0.02. We find
that such result is not improved by other lossless compressors, being the
signal almost white noise dominated. Lossless compression algorithms alone will
not solve the bandwidth problem but needs to be combined with other techniques.Comment: May 3, 2000 release, 61 pages, 6 figures coded as eps, 9 tables (4
included as eps), LaTeX 2.09 + assms4.sty, style file included, submitted for
the pubblication on PASP May 3, 200
Asteroid detection at millimetric wavelengths with the Planck survey
The Planck mission, originally devised for cosmological studies, offers the
opportunity to observe Solar System objects at millimetric and submillimetric
wavelengths. We concentrate in this paper on the asteroids of the Main Belt. We
intend to estimate the number of asteroids that can can be detected during the
mission and to evaluate the strength of their signal. We have rescaled the
instrument sensitivities, calculated by the LFI and HFI teams for sources fixed
in the sky, introducing some degradation factors to properly account for moving
objects. In this way a detection threshold is derived for asteroidal detection
that is related to the diameter of the asteroid and its geocentric distance. We
have developed a numerical code that models the detection of asteroids in the
LFI and HFI channels during the mission. This code perfoprms a detailed
integration of the orbits of the asteroids in the timespan of the mission and
identifies those bodies that fall in the beams of Planck and their signal
stenght. According to our simulations, a total of 397 objects will be observed
by Planck and an asteroidal body will be detected in some beam in 30% of the
total sky scan--circles. A significant fraction (in the range from ~50 to 100
objects) of the 397 asteroids will be observed with a high S/N ratio. Flux
measurements of a large sample of asteroids in the submillimeter and millimeter
range are relevant since they allow to analyze the thermal emission and its
relation to the surface and regolith properties. Furthermore, it will be
possible to check on a wider base the two standard thermal models, based on a
nonrotating or rapidly rotating sphere. Our method can also be used to separate
Solar System sources from cosmological sources in the survey. This work is
based on Planck LFI activities.Comment: Contact person [email protected]. Accepted for pubblication in
New Astronomy (2002). 1 figure in .eps format. Needs elsart.cls style +
harvard.st
Trade-off between angular resolution and straylight contamination in CMB anisotropy experiments. II. Straylight evaluation
Satellite CMB anisotropy missions and new generation of balloon-borne and
ground experiments, make use of complex multi-frequency instruments at the
focus of a meter class telescope. Between 70 GHz and 300 GHz, where foreground
contamination is minimum, it is extremely important to reach the best trade-off
between the improvement of the angular resolution and the minimization of the
straylight contamination mainly due to the Galactic emission. We focus here, as
a working case, on the 30 and 100 GHz channels of the Planck Low Frequency
Instrument (LFI). We evaluate the GSC introduced by the most relevant Galactic
foreground components for a reference set of optical configurations. We show
that it is possible to improve the angular resolution of 5-7% by keeping the
overall GSC below the level of few microKelvin. A comparison between the level
of straylight introduced by the different Galactic components for different
beam regions is presented. Simple approximate relations giving the rms and
peak-to-peak levels of the GSC are provided. We compare the results obtained at
100 GHz with those at 30 GHz, where GSC is more critical. Finally, we compare
the results based on Galactic foreground templates derived from radio and IR
surveys with those based on WMAP maps including CMB and extragalactic source
fluctuations.Comment: Submitted to A&A. Quality of the figures was degraded for
size-related reason
Polarization fluctuations due to extragalactic sources
We have derived the relationship between polarization and intensity
fluctuations due to point sources. In the case of a Poisson distribution of a
population with uniform evolution properties and constant polarization degree,
polarization fluctuations are simply equal to intensity fluctuations times the
average polarization degree. Conservative estimates of the polarization degree
of the classes of extragalactic sources contributing to fluctuations in the
frequency ranges covered by the forthcoming space missions MAP and Planck
Surveyor indicate that extragalactic sources will not be a strong limiting
factor to measurements of the polarization of the Cosmic Microwave Background.Comment: 15 pages LaTeX file, 3 postscript figures. Uses elsart.sty and
elsart.cls Accepted for publication in New Astronom
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