313 research outputs found
Representations of celestial coordinates in FITS
In Paper I, Greisen & Calabretta (2002) describe a generalized method for
assigning physical coordinates to FITS image pixels. This paper implements this
method for all spherical map projections likely to be of interest in astronomy.
The new methods encompass existing informal FITS spherical coordinate
conventions and translations from them are described. Detailed examples of
header interpretation and construction are given.Comment: Consequent to Paper I: "Representations of world coordinates in
FITS". 45 pages, 38 figures, 13 tables, aa macros v5.2 (2002/Jun). Both
papers submitted to Astronomy & Astrophysics (2002/07/19). Replaced to try to
get figure and table placement right (no textual changes
The On The Fly Imaging Technique
The On-The-Fly (OTF) imaging technique enables single-dish radio telescopes
to construct images of small areas of the sky with greater efficiency and
accuracy. This paper describes the practical application of the OTF imaging
technique. By way of example the implementation of the OTF imaging technique at
the NRAO 12 Meter Telescope is described. Specific requirements for data
sampling, image formation, and Doppler correction are discussed.Comment: 10 pages, 13 figures, accepted A&
Representations of world coordinates in FITS
The initial descriptions of the FITS format provided a simplified method for
describing the physical coordinate values of the image pixels, but deliberately
did not specify any of the detailed conventions required to convey the
complexities of actual image coordinates. Building on conventions in wide use
within astronomy, this paper proposes general extensions to the original
methods for describing the world coordinates of FITS data. In subsequent
papers, we apply these general conventions to the methods by which spherical
coordinates may be projected onto a two-dimensional plane and to
frequency/wavelength/velocity coordinates.Comment: 15 Pages, 1 figure, LaTex with Astronomy & Astrophysics macro
package, submitted to A&A, related papers at
http://www.aoc.nrao.edu/~egreise
The Deep SWIRE Field II. 90cm Continuum Observations and 20cm-90cm Spectra
We present one of the deepest radio continuum surveys to date at a wavelength
~1 meter, at 324.5 MHz. The data reduction and analysis are described and an
electronic catalog of the sources detected above 5 sigma is presented. We also
discuss the observed angular size distribution for the sample. Using our deeper
20cm survey of the same field, we calculate spectral indices for sources
detected in both surveys. The spectral indices for 90cm-selected sources,
defined as S ~nu^(-alpha}, shows a peak near 0.7 and only a few sources with
very steep spectra. Thus no large population of very steep spectrum microJy
sources seems to exist down to the limit of our survey.
For 20cm-selected sources, we find similar mean spectral indices for sources
with S_20>1 mJy. For weaker sources, below the detection limit for individual
sources at 90cm, we use stacking to study the radio spectra. We find that the
spectral indices of small (<3") 20cm-selected sources with S_20< 10 mJy have
mean and median alpha(90,20)~0.3-0.5. This is flatter than the spectral indices
of the stronger source population.
We report log N-log S counts at 90cm which show a flattening below 5 mJy.
Given the median redshift of the population, z~1, the spectral flattening and
the flattening of the log N-log S counts occurs at radio luminosities normally
associated with AGN rather than with galaxies dominated by star-formation.Comment: 13 pages, 8 tables, 7 figures, full electronic tables at
http://www.aoc.nrao.edu/~fowen/papers/SWIRE/90cmpaper2/, accepted A
Representations of spectral coordinates in FITS
Greisen & Calabretta describe a generalized method for specifying the
coordinates of FITS data samples. Following that general method, Calabretta &
Greisen describe detailed conventions for defining celestial coordinates as
they are projected onto a two-dimensional plane. The present paper extends the
discussion to the spectral coordinates of wavelength, frequency, and velocity.
World coordinate functions are defined for spectral axes sampled linearly in
wavelength, frequency, or velocity, linearly in the logarithm of wavelength or
frequency, as projected by ideal dispersing elements, and as specified by a
lookup table.Comment: 25 pages, 5 figure
Galactic Anisotropy as Signature of ``Top-Down'' Mechanisms of Ultra-High Energy Cosmic Rays
We show that ``top-down'' mechanisms of Ultra-High Energy Cosmic Rays which
involve heavy relic particle-like objects predict Galactic anisotropy of
highest energy cosmic rays at the level of minimum . This anisotropy
is large enough to be either observed or ruled out in the next generation of
experiments.Comment: 8 pages, 1 figure, LaTeX. Final version appeared in Pisma Zh. Eksp.
Teor. Fi
On the Origin of the Highest Energy Cosmic Rays
We present the results of a new estimation of the photodisintegration and
propagation of ultrahigh energy cosmic ray (UHCR) nuclei in intergalactic
space. The critical interactions for photodisintegration and energy loss of
UHCR nuclei occur with photons of the infrared background radiation (IBR). We
have reexamined this problem making use of a new determination of the IBR based
on empirical data, primarily from IRAS galaxies, and also collateral
information from TeV gamma-ray observations of two nearby BL Lac objects. Our
results indicate that a 200 EeV Fe nucleus can propagate apx. 100 Mpc through
the IBR. We argue that it is possible that the highest energy cosmic rays
observed may be heavy nuclei.Comment: 2 pages revtex with one figure, submitted to Physical Review Letter
Statistics of clustering of ultra-high energy cosmic rays and the number of their sources
Observation of clustering of ultra-high energy cosmic rays (UHECR) suggests
that they are emitted by compact sources. Assuming small deflection of UHECR
during the propagation, the statistical analysis of clustering allows to
estimate the spatial density of the sources, h, including those which have not
yet been observed directly. When applied to astrophysical models involving
extra-galactic sources, the estimate based on 14 events with energy E>10^{20}
eV gives h ~ 6 X 10^{-3} Mps^{-3}. With increasing statistics, this estimate
may lead to exclusion of the models which associate the production of UHECR
with exceptional galaxies such as AGN, powerful radio-galaxies, dead quasars,
and models based on gamma ray bursts.Comment: The version accepted for publication in Phys. Rev. Lett. Notations
changed to conventional ones. The estimate of the effective GZK radius
replaced by the result of numerical simulatio
Natural Candidates for Superheavy Dark Matter in String and M Theory
We reconsider superheavy dark matter candidates in string and M theory, in
view of the possibility that inflation might generate superheavy particles with
an abundance close to that required for a near-critical Universe. We argue that
cryptons - stable or metastable bound states of matter in the hidden sector -
are favoured over other possible candidates in string or theory, such as
the Kaluza-Klein states associated with extra dimensions. We exhibit a specific
string model that predicts cryptons as hidden-sector bound states weighing
GeV, and discuss their astrophysical observability.Comment: 4 pages, revtex, no figur
The first accurate parallax distance to a black hole
Using astrometric VLBI observations, we have determined the parallax of the
black hole X-ray binary V404 Cyg to be 0.418 +/- 0.024 milliarcseconds,
corresponding to a distance of 2.39 +/- 0.14 kpc, significantly lower than the
previously accepted value. This model-independent estimate is the most accurate
distance to a Galactic stellar-mass black hole measured to date. With this new
distance, we confirm that the source was not super-Eddington during its 1989
outburst. The fitted distance and proper motion imply that the black hole in
this system likely formed in a supernova, with the peculiar velocity being
consistent with a recoil (Blaauw) kick. The size of the quiescent jets inferred
to exist in this system is less than 1.4 AU at 22 GHz. Astrometric observations
of a larger sample of such systems would provide useful insights into the
formation and properties of accreting stellar-mass black holes.Comment: Accepted for publication in ApJ Letters. 6 pages, 2 figure
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