1,410 research outputs found
Unbiased flux calibration methods for spectral-line radio observations
Position and frequency switching techniques used for the removal of the
bandpass dependence of radio astronomical spectra are presented and discussed
in detail. Both methods are widely used, although the frequency dependence of
the system temperature and/or noise diode is often neglected. This leads to
systematic errors in the calibration that potentially have a significant impact
on scientific results, especially when using large-bandwidth receivers or
performing statistical analyses. We present methods to derive an unbiased
calibration using a noise diode, which is part of many heterodyne receivers. We
compare the proposed methods and describe the advantages and bottlenecks of the
various approaches. Monte Carlo simulations are used to qualitatively
investigate both systematics and the error distribution of the reconstructed
flux estimates about the correct flux values for the new methods but also the
'classical' case. Finally, the determination of the frequency-dependent noise
temperature of the calibration diode using hot-cold measurements or
observations of well-known continuum sources is also briefly discussed.Comment: 25 pages, 30 figures. Accepted for publication in A&
A Search for Active Galactic Nuclei in Sc Galaxies with H II Spectra
(Abridged) We have searched for nuclear radio emission from a statistically
complete sample of 40 Sc galaxies within 30 Mpc that are optically classified
as star-forming objects, in order to determine whether weak AGNs might be
present. Only three nuclear radio sources were detected, in NGC 864, NGC 4123,
and NGC 4535. These galaxies have peak 6-cm radio powers of 10^{20} W/Hz at
arcsecond resolution, while upper limits of the non-detected galaxies typically
range from 10^{18.4} to 10^{20} W/Hz. The three nuclear radio sources all are
resolved and appear to have diffuse morphologies, with linear sizes of ~300 pc.
This strongly indicates that circumnuclear star formation has been detected in
these three H II galaxies. Comparison with previous 20-cm VLA results for the
detected galaxies shows that the extended nuclear radio emission has a flat
spectrum in two objects, and almost certainly is generated by thermal emission
from gas ionized by young stars in the centers of those galaxies. The 6-cm
radio powers are comparable to predictions for thermal emission that are based
on the nuclear H-alpha luminosities, and imply nuclear star formation rates of
0.08-0.8 solar masses/yr, while the low-resolution NRAO VLA Sky Survey implies
galaxy-wide star formation rates of 0.3-1.0 solar masses/yr in stars above 5
solar masses. Although the presence of active nuclei powered by massive black
holes cannot be definitively ruled out, the present results suggest that they
are likely to be rare in these late-type galaxies with H II spectra.Comment: To appear in ApJ. 7 page
Skin problems of the stump and hand function in lower limb amputees:A historic cohort study
The aim of this study was to investigate the relationship between liner-related skin problems of the stump in patients with a lower limb amputation and impaired hand function. Sixty patients who were treated in a rehabilitation hospital from 1998-2006 were included in an historic cohort study. Data were collected concerning the amputation, skin problems of the stump, co-morbidity, hand function, the prosthesis, liner use and mobility score. The study population consisted of 50 trans-tibial and 10 knee disarticulation amputees, 43 male and 17 female, with a mean age of 62.3 years. The majority (63%) had a vascular reason for amputation. Blisters, folliculitis, rash and surface wounds on the stump were operationalized as being liner related. In patients with an impaired hand function, 70% had experienced liner-related skin problems of the stump, whereas 32% of the patients with a normal hand function had experienced skin problems (p=0.035). This study shows that impaired hand function poses an increased risk for skin problems in the amputation stump in patients with a lower limb amputation and liner use in their prosthesis.</p
A Wide Field, Low Frequency Radio Survey of the Field of M31: I. Construction and Statistical Analysis of the Source Catalog
We present here the results of a 325 MHz radio survey of M31, conducted with
the A-configuration of the Very Large Array. The survey covered an area of 7.6
deg, and a total of 405 radio sources between \la6\arcsec and 170\arcsec
in extent were mapped with a resolution of 6\arcsec and a 1 sensitivity
of 0.6 \mjyb. For each source, its morphological class, major axis
, minor axis , position angle , peak flux ,
integrated flux density , spectral index and spectral curvature
parameter were calculated. A comparison of the flux and radial
distribution -- both in the plane of the sky and in the plane of M31 -- of
these sources with those of the XMM--LSS and WENSS radio surveys revealed that
a vast majority of sources detected are background radio galaxies. As a result
of this analysis, we expect that only a few sources are intrinsic to M31. These
sources are identified and discussed in an accompanying paper.Comment: 29 pages with 4 tables and 10 figures (JPEGs), accepted for
publication in ApJS. Full-resolution images available on reques
Radioactivity and Electron Acceleration in Supernova Remnants
We argue that the decays of radioactive nuclei related to Ti and
Ni ejected during supernova explosions can provide a vast pool of mildly
relativistic positrons and electrons which are further accelerated to
ultrarelativistic energies by reverse and forward shocks. This interesting link
between two independent processes - the radioactivity and the particle
acceleration - can be a clue for solution of the well known theoretical problem
of electron injection in supernova remnants. In the case of the brightest radio
source Cas A, we demonstrate that the radioactivity can supply adequate number
of energetic electrons and positrons for interpretation of observational data
provided that they are stochastically pre-accelerated in the upstream regions
of the forward and reverse shocks.Comment: 6 pages, 1 figure, revised version accepted to Phys.Rev.
Filling in the Gaps in the 4.85 GHz Sky
We describe a 4.85 GHz survey of bright, flat-spectrum radio sources
conducted with the Effelsberg 100 m telescope in an attempt to improve the
completeness of existing surveys, such as CRATES. We report the results of
these observations and of follow-up 8.4 GHz observations with the VLA of a
subset of the sample. We comment on the connection to the WMAP point source
catalog and on the survey's effectiveness at supplementing the CRATES sky
coverage.Comment: 13 pages, 3 figures, 2 tables. Accepted for publication in the
Astronomical Journal. Tables available in electronic form:
http://astro.stanford.edu/gaps
A new perspective on GCRT J1745-3009
Two WSRT observations were performed and five archival VLA data were reduced
in order to redetect the enigmatic radio transient GCRT J1745-3009. The source
was not redetected. We were, however, able to extract important new information
from the discovery dataset. Our reanalysis excludes models that predict
symmetric bursts, but the transient white dwarf pulsar is favoured. Although we
now have more contraints on the properties of this source, we are still unsure
about its basic model.Comment: 11 pages, 5 figure
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