472 research outputs found
The galactic magnetic field in the quasar 3C216
Multifrequency polarimetric observations made with the Very Long Baseline
Array of the quasar 3C216 reveal the presence of Faraday rotation measures
(RMs) in excess of 2000 rad/m**2 in the source rest frame, in the arc of
emission located at ~ 140 mas from the core. Rotation measures in the range
-300 - +300 rad/m**2 are detected in the inner 5 mas (~30 parsecs). while the
rotation measures near the core can be explained as due to a magnetic field in
the narrow line region, we favor the interpretation for the high RM in the arc
as due to a ``local'' Faraday screen, produced in a shock where the jet is
deflected by the interstellar medium of the host galaxy. Our results indicate
that a galacit magnetic field of the order of 50 microGauss on a scale greater
than 100 pc must be present in the galactic medium.Comment: 23 pages, 3 tables, 11 figures. To appear on The Astronomical
Journal, November 1999 Issu
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
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
Radio Loud and Radio Quiet Active Galactic Nuclei
We generated a sample of 409 AGNs for which both the radio luminosity at 5
GHz and the line luminosity in [OIII] 5007 have been measured. The radio
luminosity spans a range of ten orders of magnitude, and the [OIII] line
luminosity spans a range of eight orders of magnitude --- both considerably
larger than the ranges in previous studies. We show that these two quantities
are correlated in a similar way for both radio-loud and radio-quiet AGNs. We
demonstrate that the observed correlation can be explained in terms of a model
in which jets are accelerated and collimated by a vertical magnetic field.Comment: 45 pages inc. 7 figures, 1 table of 15 pages in ps-format. Accept to
AJ September 199
Replication Study of Candidate Genes Associated With Type 2 Diabetes Based On Genome-Wide Screening
OBJECTIVE—The present study was conducted to confirm possible associations between candidate genes from genome-wide association studies and type 2 diabetes in Japanese diabetic patients and a community-based general population. A total of 11 previously reported single-nucleotide polymorphisms (SNPs) from the TCF7L2, CDKAL1, HHEX, IGF2BP2, CDKN2A/B, SLC30A8, and KCNJ11 genes were analyzed
Radio spectra and polarisation properties of radio-loud Broad Absorption Line Quasars
We present multi-frequency observations of a sample of 15 radio-emitting
Broad Absorption Line Quasars (BAL QSOs), covering a spectral range between 74
MHz and 43 GHz. They display mostly convex radio spectra which typically peak
at about 1-5 GHz (in the observer's rest-frame), flatten at MHz frequencies,
probably due to synchrotron self-absorption, and become steeper at high
frequencies, i.e., >~ 20 GHz. VLA 22-GHz maps (HPBW ~ 80 mas) show unresolved
or very compact sources, with linear projected sizes of <= 1 kpc. About 2/3 of
the sample look unpolarised or weakly polarised at 8.4 GHz, frequency in which
reasonable upper limits could be obtained for polarised intensity. Statistical
comparisons have been made between the spectral index distributions of samples
of BAL and non-BAL QSOs, both in the observed and the rest-frame, finding
steeper spectra among non-BAL QSOs. However constraining this comparison to
compact sources results in no significant differences between both
distributions. This comparison is consistent with BAL QSOs not being oriented
along a particular line of sight. In addition, our analysis of the spectral
shape, variability and polarisation properties shows that radio BAL QSOs share
several properties common to young radio sources like Compact Steep Spectrum
(CSS) or Gigahertz-Peaked Spectrum (GPS) sources.Comment: 18 pages, 11 Postscript figures, 12 Tables. Accepted for publication
in MNRA
Milestones in the Observations of Cosmic Magnetic Fields
Magnetic fields are observed everywhere in the universe. In this review, we
concentrate on the observational aspects of the magnetic fields of Galactic and
extragalactic objects. Readers can follow the milestones in the observations of
cosmic magnetic fields obtained from the most important tracers of magnetic
fields, namely, the star-light polarization, the Zeeman effect, the rotation
measures (RMs, hereafter) of extragalactic radio sources, the pulsar RMs, radio
polarization observations, as well as the newly implemented sub-mm and mm
polarization capabilities.
(Another long paragraph is omitted due to the limited space here)Comment: Invited Review (ChJA&A); 32 pages. Sorry if your significant
contributions in this area were not mentioned. Published pdf & ps files (with
high quality figures) now availble at http://www.chjaa.org/2002_2_4.ht
High-resolution radio continuum survey of M33: III. Magnetic fields
Using the linearly polarized intensity and polarization angle data at 3.6,
6.2 and 20 cm, we determine variations of Faraday rotation and depolarization
across the nearby galaxy M33. A 3-D model of the regular magnetic field is
fitted to the observed azimuthal distribution of polarization angles. Faraday
rotation, measured between 3.6 and 6.2 cm at a linear resolution of 0.7 kpc,
shows more variation in the south than in the north of the galaxy. About 10% of
the nonthermal emission from M33 at 3.6 cm is polarized. We estimate the
average total and regular magnetic field strengths in M33 as ~ 6.4 and 2.5
G, respectively. Under the assumption that the disk of M33 is flat, the
regular magnetic field consists of horizontal and vertical components: however
the inferred vertical field may be partly due to a galactic warp. The
horizontal field is represented by an axisymmetric (m=0) mode from 1 to 3 kpc
radius and a superposition of axisymmetric and bisymmetric (m=0+1) modes from 3
to 5 kpc radius. An excess of differential Faraday rotation in the southern
half together with strong Faraday dispersion in the southern spiral arms seem
to be responsible for the north-south asymmetry in the observed wavelength
dependent depolarization. The presence of an axisymmetric m=0 mode of the
regular magnetic field in each ring suggests that a galactic dynamo is
operating in M33. The pitch angles of the spiral regular magnetic field are
generally smaller than the pitch angles of the optical spiral arms but are
twice as big as simple estimates based on the mean-field dynamo theory and
M33's rotation curve. Generation of interstellar magnetic fields from turbulent
gas motions in M33 is indicated by the equipartition of turbulent and magnetic
energy densities.Comment: 17 pages, 12 figures, accepted for Astronomy and Astrophysics
publicatio
FTO, Type 2 Diabetes, and Weight Gain Throughout Adult Life: A Meta-Analysis of 41,504 Subjects From the Scandinavian HUNT, MDC, and MPP Studies
OBJECTIVE—FTO is the most important polygene identified for obesity. We aimed to investigate whether a variant in FTO affects type 2 diabetes risk entirely through its effect on BMI and how FTO influences BMI across adult life span. RESEARCH DESIGN AND METHODS—Through regression models, we assessed the relationship between the FTO single nucleotide polymorphisms rs9939609, type 2 diabetes, and BMI across life span in subjects from the Norwegian population-based HUNT study using cross-sectional and longitudinal perspectives. For replication and meta-analysis, we used data from the Malmö Diet and Cancer (MDC) and Malmö Preventive Project (MPP) cohorts, comprising a total sample of 41,504 Scandinavians.RESULTS—The meta-analysis revealed a highly significant association for rs9939609 with both type 2 diabetes (OR 1.13; P = 4.5 3 1028) and the risk to develop incident type 2 diabetes (OR 1.16; P = 3.2 3 1028). The associations remained also after correction for BMI and other anthropometric measures. Furthermore, we confirmed the strong effect on BMI (0.28 kg/m2 per risk allele; P = 2.0 3 10226), with no heterogeneity between different age-groups. We found no differences in change of BMI over time according to rs9939609 risk alleles, neither overall (ΔBMI = 0.0[20.05, 0.05]) nor in any individual age stratum, indicating no further weight gain attributable to FTO genotype in adults. CONCLUSIONS—We have identified that a variant in FTO alters type 2 diabetes risk partly independent of its observed effect on BMI. The additional weight gain as a result of the FTO risk variant seems to occur before adulthood, and the BMI difference remains stable thereafter
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