533 research outputs found
Udder conformation and its heritability in the Assaf (Awassi Ă— East Friesian) cross of dairy sheep in Israel
International audienc
Compact Structure in FIRST Survey Sources
We present preliminary results from a statistical survey of compact structure
in faint radio sources. Around 1000 sources from the VLA FIRST survey (flux
densities larger than 1 mJy at 1.4 GHz) have been observed with the
single-baseline interferometer Effelsberg-Arecibo. We observed each source,
selected from a narrow strip of sky at declination 28 deg, for just one minute.
The baseline sensitivity at 1.4 GHz, using 512 Mb/s recording, is such that any
FIRST source, selected at random, would be detected if most of its flux density
is in compact structure. We discuss the detection-rate statistics from one
epoch of these observations.Comment: 4 pages. 12 figures. Proceedings of the 7th European VLBI Network
Symposium held in Toledo, Spain on October 12-15, 2004. Editors: R.
Bachiller, F. Colomer, J.-F. Desmurs, P. de Vicente (Observatorio Astronomico
Nacional), p. 31-34. Needs evn2004.cl
Evidence for Nonlinear X-ray Variability from the Broad-line Radio Galaxy 3C 390.3
We present analysis of the light curve from the ROSAT HRI monitoring
observations of the broad-line radio galaxy 3C 390.3. Observed every three days
for about 9 months, this is the first well sampled X-ray light curve on these
time scales. The flares and quiescent periods in the light curve suggest that
the variability is nonlinear, and a statistical test yields a detection with >6
sigma confidence. The structure function has a steep slope ~0.7, while the
periodogram is much steeper with a slope ~2.6, with the difference partially
due to a linear trend in the data. The non-stationary character of the light
curve could be evidence that the variability power spectrum has not turned over
to low frequencies, or it could be an essential part of the nonlinear process.
Evidence for X-ray reprocessing suggests that the X-ray emission is not from
the compact radio jet, and the reduced variability before and after flares
suggests there cannot be two components contributing to the X-ray short term
variability. Thus, these results cannot be explained easily by simple models
for AGN variability, including shot noise which may be associated with flares
in disk-corona models or active regions on a rotating disk, because in those
models the events are independent and the variability is therefore linear. The
character of the variability is similar to that seen in Cygnus X-1, which has
been explained by a reservoir or self-organized criticality model. Inherently
nonlinear, this model can reproduce the reduced variability before and after
large flares and the steep PDS seen generally from AGN. The 3C 390.3 light
curve presented here is the first support for such models to explain AGN
variability on intermediate time scales from a few days to months.Comment: 10 pages using (AASTeX) aaspp4.sty and 3 Postscript figures.
Astrophysical Journal Letters, in pres
DiFX2: A more flexible, efficient, robust and powerful software correlator
Software correlation, where a correlation algorithm written in a high-level
language such as C++ is run on commodity computer hardware, has become
increasingly attractive for small to medium sized and/or bandwidth constrained
radio interferometers. In particular, many long baseline arrays (which
typically have fewer than 20 elements and are restricted in observing bandwidth
by costly recording hardware and media) have utilized software correlators for
rapid, cost-effective correlator upgrades to allow compatibility with new,
wider bandwidth recording systems and improve correlator flexibility. The DiFX
correlator, made publicly available in 2007, has been a popular choice in such
upgrades and is now used for production correlation by a number of
observatories and research groups worldwide. Here we describe the evolution in
the capabilities of the DiFX correlator over the past three years, including a
number of new capabilities, substantial performance improvements, and a large
amount of supporting infrastructure to ease use of the code. New capabilities
include the ability to correlate a large number of phase centers in a single
correlation pass, the extraction of phase calibration tones, correlation of
disparate but overlapping sub-bands, the production of rapidly sampled
filterbank and kurtosis data at minimal cost, and many more. The latest version
of the code is at least 15% faster than the original, and in certain situations
many times this value. Finally, we also present detailed test results
validating the correctness of the new code.Comment: 28 pages, 9 figures, accepted for publication in PAS
870 micron observations of nearby 3CRR radio galaxies
We present submillimeter continuum observations at 870 microns of the cores
of low redshift 3CRR radio galaxies, observed at the Heinrich Hertz
Submillimeter Telescope. The cores are nearly flat spectrum between the radio
and submillimeter which implies that the submillimeter continuum is likely to
be synchrotron emission and not thermal emission from dust. The emitted power
from nuclei detected at optical wavelengths and in the X-rays is similar in the
submillimeter, optical and X-rays. The submillimeter to optical and X-ray power
ratios suggest that most of these sources resemble misdirected BL Lac type
objects with synchrotron emission peaking at low energies. However we find
three exceptions, the FR I galaxy 3C264 and the FR II galaxies 3C390.3 and
3C338 with high X-ray to submillimeter luminosity ratios. These three objects
are candidate high or intermediate energy peaked BL Lac type objects. With
additional infrared observations and from archival data, we compile spectral
energy distributions (SEDs) for a subset of these objects. The steep dips
observed near the optical wavelengths in many of these objects suggest that
extinction inhibits the detection and reduces the flux of optical continuum
core counterparts. High resolution near or mid-infrared imaging may provide
better measurements of the underlying synchrotron emission peak.Comment: accepted for publication in A
On the Calibration of Full-polarization 86GHz Global VLBI Observations
We report the development of a semi-automatic pipeline for the calibration of
86 GHz full-polarization observations performed with the Global Millimeter-VLBI
array (GMVA) and describe the calibration strategy followed in the data
reduction. Our calibration pipeline involves non-standard procedures, since
VLBI polarimetry at frequencies above 43 GHz is not yet well established. We
also present, for the first time, a full-polarization global-VLBI image at 86
GHz (source 3C 345), as an example of the final product of our calibration
pipeline, and discuss the effect of instrumental limitations on the fidelity of
the polarization images. Our calibration strategy is not exclusive for the
GMVA, and could be applied on other VLBI arrays at millimeter wavelengths. The
use of this pipeline will allow GMVA observers to get fully-calibrated datasets
shortly after the data correlation.Comment: 10 pages, 10 figures. Accepted for publication in A&
Zooming towards the Event Horizon - mm-VLBI today and tomorrow
Global VLBI imaging at millimeter and sub-millimeter wavelength overcomes the
opacity barrier of synchrotron self-absorption in AGN and opens the direct view
into sub-pc scale regions not accessible before. Since AGN variability is more
pronounced at short millimeter wavelength, mm-VLBI can reveal structural
changes in very early stages after outbursts. When combined with observations
at longer wavelength, global 3mm and 1mm VLBI adds very detailed information.
This helps to determine fundamental physical properties at the jet base, and in
the vicinity of super-massive black holes at the center of AGN. Here we present
new results from multi-frequency mm-VLBI imaging of OJ287 during a major
outburst. We also report on a successful 1.3mm VLBI experiment with the APEX
telescope in Chile. This observation sets a new record in angular resolution.
It also opens the path towards future mm-VLBI with ALMA, which aims at the
mapping of the black hole event horizon in nearby galaxies, and the study of
the roots of jets in AGN.Comment: 6 pages, to appear in 11th European VLBI Network Symposium, ed. P.
Charlot et al., Bordeaux (France), October 9-12, 201
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