48,883 research outputs found
Measuring and calibrating Galactic synchrotron emission
Our position inside the Galaxy requires all-sky surveys to reveal its
large-scale properties. The zero-level calibration of all-sky surveys differs
from standard 'relative' measurements, where a source is measured in respect to
its surroundings. All-sky surveys aim to include emission structures of all
angular scales exceeding their angular resolution including isotropic emission
components. Synchrotron radiation is the dominating emission process in the
Galaxy up to frequencies of a few GHz, where numerous ground based surveys of
the total intensity up to 1.4 GHz exist. Its polarization properties were just
recently mapped for the entire sky at 1.4 GHz. All-sky total intensity and
linear polarization maps from WMAP for frequencies of 23 GHz and higher became
available and complement existing sky maps. Galactic plane surveys have higher
angular resolution using large single-dish or synthesis telescopes. Polarized
diffuse emission shows structures with no relation to total intensity emission
resulting from Faraday rotation effects in the interstellar medium. The
interpretation of these polarization structures critically depends on a correct
setting of the absolute zero-level in Stokes U and Q.Comment: 10 pages, 8 figures. To be published in "Cosmic Magnetic Fields: From
Planets, to Stars and Galaxies", K.G. Strassmeier, A.G. Kosovichev & J.E.
Beckman, eds., Proc. IAU Symp. 259, CU
A statistical analysis of a Galactic all sky survey at 1.4 GHz
Radio surveys at frequencies of about 1 GHz allow to map the synchrotron
emission in a frequency range where (except for very low Galactic latitudes or
towards localized regions) it dominates over the other radio components. New
all sky total intensity and polarization data at 1.4 GHz have been recently
collected. We focus on the Galactic radio emission correlation properties
described in terms of angular power spectrum (APS). We present for the first
time the APS, in both total intensity and polarization modes, for some
representative Galactic cuts and suitable APS power law parametrizations.Comment: Comments: 2 pages, 2 figures; in Astronomische Nachrichten, Vol.327,
Issue 5/6, p.491 (2006); Proceedings of International Conference "The Origin
and Evolution of Cosmic Magnetism", 29 August - 2 September 2005, CNR Area
della Ricerca, Bologna, Italy, eds. R. Beck, G. Brunetti, L. Feretti, and B.
Gaensle
Exciton resonances quench the photoluminescence of zigzag carbon nanotubes
We show that the photoluminescence intensity of single-walled carbon
nanotubes is much stronger in tubes with large chiral angles - armchair tubes -
because exciton resonances make the luminescence of zigzag tubes intrinsically
weak. This exciton-exciton resonance depends on the electronic structure of the
tubes and is found more often in nanotubes of the +1 family. Armchair tubes do
not necessarily grow preferentially with present growth techniques; they just
have stronger luminescence. Our analysis allows to normalize photoluminescence
intensities and find the abundance of nanotube chiralities in macroscopic
samples.Comment: 4 pages and 2 supplementary pages; 6 figure
Question/answer congruence and the semantics of wh-phrases
This paper is about the semantics of wh-phrases. It is argued that wh-phrases should not be analyzed as indefinites as, for example, Karttunen (1977) and many others have done, but as functional expressions with an indefinite core -their function being to restrict possible focus/background structures in direct or congruent answers. This will be argued for on the basis of observations made with respect to the distribution of term answers in well-formed question/answer sequences. This claim having been established, it will be integrated in a categorial variant of Schwarzschild's (1999) information-theoretic approach to F-marking and accent placement, and – second – its consequences with respect to the focus/background structure of wh-questions will be outlined
Radio Observations of Supernova Remnants
Supernovae release an enormous amount of energy into the interstellar medium.
Their remnants can observationally be traced up to several ten-thousand years.
So far more than 230 Galactic supernova remnants (SNRs) have been identified in
the radio range. Detailed studies of the different types of SNRs give insight
into the interaction of the blast wave with the interstellar medium. Shock
accelerated particles are observed, but also neutron stars left from the
supernova explosion make their contribution. X-ray observations in conjunction
with radio data constrain models of supernova evolution.
A brief review of the origin and evolution of SNRs is given, which are
compared with supernova statistics and observational limitations. In addition
the morphology and characteristics of the different types of SNRs are
described, including some recent results and illustrated by SNRs images mostly
obtained with the Effelsberg 100-m telescope.Comment: 11 pages, 15 figures. To appear in the Proceedings of the 270.
WE-Heraeus Seminar on Neutron Stars, Pulsars and Supernova Remnants, Jan.
21-25, 2002, Physikzentrum Bad Honnef, eds W. Becker, H. Lesch & J. Truemper.
Proceedings are available as MPE-Report 27
A radio continuum survey of the southern sky at 1420 MHz. Observations and data reduction
We describe the equipment, observational method and reduction procedure of an
absolutely calibrated radio continuum survey of the South Celestial Hemisphere
at a frequency of 1420 MHz. These observations cover the area 0h < R.A. < 24h
for declinations less than -10 degree. The sensitivity is about 50 mK T_B (full
beam brightness) and the angular resolution (HPBW) is 35.4', which matches the
existing northern sky survey at the same frequency.Comment: 9 pages with 9 figures, A&A, in pres
G181.1+9.5, a new high-latitude low-surface brightness supernova remnant
More than 90% of the known Milky Way supernova remnants are within 5 degrees
of the Galactic Plane. We present the discovery of the supernova remnant
G181.1+9.5, a new high-latitude SNR, serendipitously discovered in an ongoing
survey of the Galactic Anti-centre High-Velocity Cloud complex, observed with
the DRAO Synthesis Telescope in the 21~cm radio continuum and HI spectral line.
We use radio continuum observations (including the linearly polarized
component) at 1420~MHz (observed with the DRAO ST) and 4850~MHz (observed with
the Effelsberg 100-m radio telescope) to map G181.1+9.5 and determine its
nature as a SNR. High-resolution 21~cm HI line observations and HI emission and
absorption spectra reveal the physical characteristics of its local
interstellar environment. Finally, we estimate the basic physical parameters of
G181.1+9.5 using models for highly-evolved SNRs. G181.1+9.5 has a circular
shell-like morphology with a radius of about 16~pc at a distance of 1.5 kpc
some 250 pc above the mid-plane. The radio observations reveal highly linearly
polarized emission with a non-thermal spectrum. Archival ROSAT X-ray data
reveal high-energy emission from the interior of G181.1+9.5 indicative of the
presence of shock-heated ejecta. The SNR is in the advanced radiative phase of
SNR evolution, expanding into the HVC inter-cloud medium with a density of
1^{-3}$. Basic physical attributes of G181.1+9.5 calculated with radiative
SNR models show an upper-limit age of 16,000 years, a swept-up mass of more
than 300 solar masses, and an ambient density in agreement with that estimated
from HI observations. G181.1+9.5 shows all characteristics of a typical mature
shell-type SNR, but its observed faintness is unusual and requires further
study.Comment: A&A accepted, 11 pages, 13 figure
Obvious natural morphisms of sheaves are unique
We prove that a large class of natural transformations (consisting roughly of
those constructed via composition from the "functorial" or "base change"
transformations) between two functors of the form
actually has only one element, and thus that any diagram of such maps
necessarily commutes. We identify the precise axioms defining what we call a
"geofibered category" that ensure that such a coherence theorem exists. Our
results apply to all the usual sheaf-theoretic contexts of algebraic geometry.
The analogous result that would include any other of the six functors remains
unknown.Comment: 52 pages. Final draft, version accepted to TA
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