153 research outputs found
An absolutely calibrated survey of polarized emission from the northern sky at 1.4 GHz
A new polarization survey of the northern sky at 1.41 GHz is presented. The
observations were carried out using the 25.6m telescope at the Dominion Radio
Astrophysical Observatory in Canada, with an angular resolution of 36 arcmin.
The data are corrected for ground radiation to obtain Stokes U and Q maps on a
well-established intensity scale tied to absolute determinations of zero
levels, containing emission structures of large angular extent, with an rms
noise of 12 mK. Survey observations were carried out by drift scanning the sky
between -29 degr and +90 degr declination. The fully sampled drift scans,
observed in steps of 0.25 degr to 2.5 degr in declination, result in a northern
sky coverage of 41.7% of full Nyquist sampling. The survey surpasses by a
factor of 200 the coverage, and by a factor of 5 the sensitivity, of the
Leiden/Dwingeloo polarization survey (Spoelstra 1972) that was until now the
most complete large-scale survey. The temperature scale is tied to the
Effelsberg scale. Absolute zero-temperature levels are taken from the
Leiden/Dwingeloo survey after rescaling those data by the factor of 0.94. The
paper describes the observations, data processing, and calibration steps. The
data are publicly available at http://www.mpifr-bonn.mpg.de/div/konti/26msurvey
or http://www.drao.nrc.ca/26msurvey.Comment: 18 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
Analysis of the thin layer of Galactic warm ionized gas in the range 20 < l < 30 deg, -1.5 < b < +1.5 deg
We present an analysis of the thin layer of Galactic warm ionized gas at an
angular resolution ~ 10'. This is carried out using radio continuum data at 1.4
GHz, 2.7 GHz and 5 GHz in the coordinate region 20 < l < 30 deg, -1.5 < b <
+1.5 deg. For this purpose, we evaluate the zero level of the 2.7 and 5 GHz
surveys using auxiliary data at 2.3 GHz and 408 MHz. The derived zero level
corrections are T_{zero}(2.7 GHz)=0.15 +/- 0.06 K and T_{zero}(5 GHz)=0.1 +/-
0.05 K. We separate the thermal (free-free) and non-thermal (synchrotron)
component by means of a spectral analysis performed adopting an antenna
temperature spectral index -2.1 for the free-free emission, a realistic spatial
distribution of indices for the synchrotron radiation and by fitting,
pixel-by-pixel, the Galactic spectral index. We find that at 5 GHz, for |b| = 0
deg, the fraction of thermal emission reaches a maximum value of 82%, while at
1.4 GHz, the corresponding value is 68%. In addition, for the thermal emission,
the analysis indicates a dominant contribution of the diffuse component
relative to the source component associated with discrete HII regions.Comment: 9 pages, 9 figures, accepted to MNRA
Properties of the warm magnetized ISM, as inferred from WSRT polarimetric imaging
We describe a first attempt to derive properties of the regular and turbulent
Galactic magnetic field from multi-frequency polarimetric observations of the
diffuse Galactic synchrotron background. A single-cell-size model of the thin
Galactic disk is constructed which includes random and regular magnetic fields
and thermal and relativistic electrons. The disk is irradiated from behind with
a uniform partially polarized background. Radiation from the background and
from the thin disk is Faraday rotated and depolarized while propagating through
the medium. The model parameters are estimated from a comparison with 350 MHz
observations in two regions at intermediate latitudes done with the Westerbork
Synthesis Radio Telescope. We obtain good consistency between the estimates for
the random and regular magnetic field strengths and typical scales of structure
in the two regions. The regular magnetic field strength found is a few
microGauss, and the ratio of random to regular magnetic field strength is 0.7
+/- 0.5, for a typical scale of the random component of 15 +/- 10 pc.
Furthermore, the regular magnetic field is directed almost perpendicular to the
line of sight. This modeling is a potentially powerful method to estimate the
structure of the Galactic magnetic field, especially when more polarimetric
observations of the diffuse synchrotron background at intermediate latitudes
become available.Comment: 12 pages, 6 figures, accepted by A&
New lambda6cm observations of the Cygnus Loop
Radio continuum and polarization observations of the entire Cygnus Loop at
6cm wavelength were made with the Urumqi 25m telescope. The 6cm map is analysed
together with recently published maps from the Effelsberg 100m telescope at
21cm and 11cm. The integrated flux density of the Cygnus Loop at 6cm is
90+/-9Jy, which implies a spectral index of -0.40+/-0.06 being consistent with
that of Uyaniker et al. (2004) in the wavelength range up to 11cm. This rules
out any global spectral steepening up to 6cm. However, small spectral index
variations in some regions of the source are possible, but there are no
indications for any spectral curvature. The linear polarization data at 6cm
show percentage polarizations up to 35% in some areas of the Cygnus Loop,
exceeding those observed at 11cm. The Rotation Measure is around -21rad/m^2 in
the southern area, which agrees with previous observations. However, the
distribution of Rotation Measures is rather complex in the northern part of the
Cygnus Loop, where the 21cm emission is totally depolarized. Rotation Measures
based on 11cm and 6cm data are significantly larger than in the southern part.
The difference in the polarization characteristic between the northern and
southern part supports previous ideas that the Cygnus Loop consists of two
supernova remnants.Comment: 11 pages, 10 figures, accepted for publication in A&
Weighting CMB and Galactic synchrotron polarisation
We review the present knowledge of the diffuse Galactic synchrotron emission
in polarisation. At microwave frequencies, we assess the expected contamination
to the CMB polarisation angular power spectrum, for and modes, as
expected after the WMAP first year measurements.Comment: 6 pages, 4 figures, proc. of the CMBnet workshop, 20-21 Feb. 2003,
Oxford, U
Radio Polarization from the Galactic Plane in Cygnus
We present 1420 MHz (lambda=21cm) observations of polarized emission from an area of 117 degree square in the Galactic plane in Cygnus, covering 82 < l < 95, -3.5 < b < +5.5, a complex region where the line of sight is directed nearly along the Local spiral arm. The angular resolution is ~ 1', and structures as large as 45' are fully represented in the images. Polarization features bear little resemblance to features detected in total power: while the polarized signal arises in diffuse Galactic synchrotron emission regions, the appearance of the polarized sky is dominated by Faraday rotation occurring in small-scale structure in the intervening Warm Ionized Medium. There is no concentration of polarization structure towards the Galactic plane, indicating that both the emission and Faraday rotation occur nearby. We develop a conceptual framework for interpretation of the observations. We can detect only that polarized emission which has its origin closer than the polarization horizon, at a distance d_ph; more distant polarized emission is undetectable because of depth depolarization (differential Faraday rotation) and/or beam depolarization (due to internal and external Faraday dispersion). d_ph depends on the instrument used (frequency and beamwidth) as well as the direction being studied. In our data we find that d_ph ~ 2 kpc, consistent with the polarization features originating in the Local arm
The SNR G106.3+2.7 and its Pulsar Wind Nebula: relics of triggered star formation in a complex environment
We propose that the pulsar nebula associated with the pulsar J2229+6114 and
the supernova remnant (SNR) G106.3+2.7 are the result of the same supernova
explosion. The whole structure is located at the edge of an HI bubble with
extended regions of molecular gas inside. The radial velocities of both the
atomic hydrogen and the molecular material suggest a distance of 800 pc. At
this distance the SNR is 14 pc long and 6 pc wide. Apparently the bubble was
created by the stellar wind and supernova explosions of a group of stars in its
center which also triggered the formation of the progenitor star of G106.3+2.7.
The progenitor star exploded at or close to the current position of the pulsar,
which is at one end of the SNR rather than at its center. The expanding shock
wave of the supernova explosion created a comet shaped supernova remnant by
running into dense material and then breaking out into the inner part of the HI
bubble. A synchrotron nebula with a shell-like structure (the ``Boomerang'') of
length 0.8 pc was created by the pulsar wind interacting with the dense ambient
medium. The expanding shock wave created an HI shell of mass 0.4 Msun around
this nebula by ionizing the atomic hydrogen in its vicinity.Comment: 10 pages, Latex, with aastex and emulateapj5, 5 figures. ApJ,
accepted, scheduled for the v560 n1 p1 Oct 10, 2001 issu
The Ku-band Polarization Identifier
The Ku-band Polarization Identifier (KUPID) will integrate a very low noise
12-18 GHz, correlation polarimeter onto the Crawford Hill seven meter,
millimeter-wave antenna. The primary components of the polarimeter will be
built at the University of Miami and other key components, including the
microwave horn and data acquisition system will be built at the University of
Chicago and Princeton University. This project will measure the Q and U Stokes
parameters in regions near the north celestial pole, in regions of low galactic
contamination, and in regions near the galactic plane. The KUPID survey
experiment makes use of many of the techniques employed in the Princeton IQU
Experiment (PIQUE) that was developed by the members of this collaboration to
detect CMB polarization at shorter wavelengths. The KUPID experiment will be
constructed in parallel and on the same timescale as the CAPMAP experiment (see
Barkats, this volume) which is the follow-on experiment to PIQUE. KUPID will
observe on the Crawford Hill antenna from late spring until early autumn, while
CAPMAP will observe during the lower water vapor months of late autumn until
early spring.Comment: To be published in the proceedings of "The Cosmic Microwave
Background and its Polarization", New Astronomy Reviews, (eds. S. Hanany and
K. A. Olive
The distance to the SNR CTB109 deduced from its environment
We conducted a study of the environment around the supernova remnant CTB109.
We found that the SNR is part of a large complex of HII regions extending over
an area of 400 pc along the Galactic plane at a distance of about 3 kpc at the
closer edge of the Perseus spiral arm. At this distance CTB109 has a diameter
of about 24 pc. We demonstrated that including spiral shocks in the distance
estimation is an ultimate requirement to determine reliable distances to
objects located in the Perseus arm. The most likely explanation for the high
concentration of HII regions and SNRs is that the star formation in this part
of the Perseus arm is triggered by the spiral shock.Comment: 6 pages, 6 figures. accepted for publication in the Astrophysical
Journa
Detection of a radio halo in the Virgo cluster
New Effelsberg 1.4 GHz observations of the central 10\degr x 10\degr of the
Virgo cluster are presented. NVSS data are used to subtract point sources from
our map. During the data reduction process special care is taken (i) to
disentangle emission from the North Polar Spur from emission from the Virgo
cluster, (ii) to disentangle emission from the strong M87 sidelobes from
emission from the Virgo cluster, and (iii) to correct for non-linear ground
emission due to the long scans. We detect a low surface brightness radio halo
with a flux density of 5 +/- 1.5 Jy centered close to the elliptical galaxy
M86. This halo is much weaker than that observed in the Coma cluster. It is
reminiscent of a past interaction between the intracluster medium of M86 and a
low density gas, belonging most probably to the Virgo cluster.Comment: 9 pages, 8 figures. Accepted for publication in A&
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