2,556 research outputs found
Hard x-ray polarimetry with the Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
Although designed primarily as a hard X-ray imager and spectrometer, the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is also capable of measuring the polarization of hard X-rays (20-100 keV) from solar flares. This capability arises from the inclusion of a small unobstructed Be scattering element that is strategically located within the cryostat that houses the array of nine germanium detectors. The Ge detectors are segmented, with both a front and rear active volume. Low energy photons (below about 100 keV) can reach a rear segment of a Ge detector only indirectly, by scattering. Low energy photons from the Sun have a direct path to the Be and have a high probability of Compton scattering into a rear segment of a Ge detector. The azimuthal distribution of these scattered photons carries with it a signature of the linear polarization of the incident flux. Sensitivity estimates, based on simulations and in-flight background measurements, indicate that a 20-100 keV polarization sensitivity of less than a few percent can be achieved for X-class flares
Idempotent ideals and non-finitely generated projective modules over integral group rings of polycyclic-by-finite groups
We prove that every non-finitely generated projective module over the
integral group ring of a polycyclic-by-finite group G is free if and only if G
is polycyclic.Comment: 15 pages, to appear in J. Algebr
Unruffled extensions and flatness over central subalgebras
A condition on an affine central subalgebra of a noetherian algebra
of finite Gelfand-Kirillov dimension, which we call here \emph{unruffledness},
is shown to be equivalent in some circumstances to the flatness of as a
-module. Unruffledness was studied by Borho and Joseph in work on enveloping
algebras of complex semisimple Lie algebras, and we discuss applications of our
result to enveloping algebras, as well as beginning the study of this condition
for more general algebras
High-velocity OH megamasers in IRAS 20100-4156: Evidence for a Supermassive Black Hole
We report the discovery of new, high-velocity narrow-line components of the
OH megamaser in IRAS 20100-4156. Results from the Australian Square Kilometre
Array Pathfinder (ASKAP)'s Boolardy Engineering Test Array (BETA) and the
Australia Telescope Compact Array (ATCA) provide two independent measurements
of the OH megamaser spectrum. We found evidence for OH megamaser clumps at
409 and 562 km/s (blue-shifted) from the systemic velocity of the galaxy,
in addition to the lines previously known. The presence of such high velocities
in the molecular emission from IRAS 201004156 could be explained by a ~50 pc
molecular ring enclosing an approximately 3.8 billion solar mass black hole. We
also discuss two alternatives, i.e. that the narrow-line masers are dynamically
coupled to the wind driven by the active galactic nucleus or they are
associated with two separate galactic nuclei. The comparison between the BETA
and ATCA spectra provides another scientific verification of ASKAP's BETA. Our
data, combined with previous measurements of the source enabled us to study the
variability of the source over a twenty-six year period. The flux density of
the brightest OH maser components has reduced by more than a factor of two
between 1988 and 2015, whereas a secondary narrow-line component has more than
doubled in the same time. Plans for high-resolution VLBI follow-up of this
source are discussed, as are prospects for discovering new OH megamasers during
the ASKAP early science program.Comment: Accepted to MNRAS. Seven pages, three figure
Single-strand nicks induce homologous recombination with less toxicity than double-strand breaks using an AAV vector template
Gene targeting by homologous recombination (HR) can be induced by double-strand breaks (DSBs), however these breaks can be toxic and potentially mutagenic. We investigated the I-AniI homing endonuclease engineered to produce only nicks, and found that nicks induce HR with both plasmid and adeno-associated virus (AAV) vector templates. The rates of nick-induced HR were lower than with DSBs (24-fold lower for plasmid transfection and 4- to 6-fold lower for AAV vector infection), but they still represented a significant increase over background (240- and 30-fold, respectively). We observed severe toxicity with the I-AniI ācleavaseā, but no evidence of toxicity with the I-AniI ānickase.ā Additionally, the frequency of nickase-induced mutations at the I-AniI site was at least 150-fold lower than that induced by the cleavase. These results, and the observation that the surrounding sequence context of a target site affects nick-induced HR but not DSB-induced HR, strongly argue that nicks induce HR through a different mechanism than DSBs, allowing for gene correction without the toxicity and mutagenic activity of DSBs
Cold gas outflows from the Small Magellanic Cloud traced with ASKAP
Feedback from massive stars plays a critical role in the evolution of the
Universe by driving powerful outflows from galaxies that enrich the
intergalactic medium and regulate star formation. An important source of
outflows may be the most numerous galaxies in the Universe: dwarf galaxies.
With small gravitational potential wells, these galaxies easily lose their
star-forming material in the presence of intense stellar feedback. Here, we
show that the nearby dwarf galaxy, the Small Magellanic Cloud (SMC), has atomic
hydrogen outflows extending at least 2 kiloparsecs (kpc) from the star-forming
bar of the galaxy. The outflows are cold, , and may have formed
during a period of active star formation million years (Myr) ago. The
total mass of atomic gas in the outflow is solar masses, , or % of the total atomic gas of the galaxy. The inferred
mass flux in atomic gas alone, , is up to an order of magnitude greater than the star
formation rate. We suggest that most of the observed outflow will be stripped
from the SMC through its interaction with its companion, the Large Magellanic
Cloud (LMC), and the Milky Way, feeding the Magellanic Stream of hydrogen
encircling the Milky Way.Comment: Published in Nature Astronomy, 29 October 2018,
http://dx.doi.org/10.1038/s41550-018-0608-
Rapid expression of chemokines and pro-inflammatory cytokines in newly hatched chickens infected with Salmonella enterica serovar Typhimurium
Field sources near the southern-sky calibrator PKS B1934-638: effect on spectral line observations with SKA-MID and its precursors
Accurate instrumental bandpass corrections are essential for the reliable
interpretation of spectral lines from targeted and survey-mode observations
with radio interferometers. Bandpass correction is typically performed by
comparing measurements of a strong calibrator source to an assumed model,
typically an isolated point source. The wide field-of-view and high sensitivity
of modern interferometers means that additional sources are often detected in
observations of calibrators. This can introduce errors into bandpass
corrections and subsequently the target data if not properly accounted for.
Focusing on the standard calibrator PKS B1934-638, we perform simulations to
asses this effect by constructing a wide-field sky model. The cases of ASKAP
(0.7-1.9 GHz), MeerKAT (UHF: 0.58-1.05 GHz; L-band: 0.87-1.67 GHz) and Band 2
(0.95-1.76 GHz) of SKA-MID are examined. The use of a central point source
model during bandpass calibration is found to impart amplitude errors into
spectra measured by the precursor instruments at the ~0.2-0.5% level dropping
to ~0.01% in the case of SKA-MID. This manifests itself as ripples in the
source spectrum, the behaviour of which is coupled to the distribution of the
array baselines, the solution interval, the primary beam size, the hour-angle
of the calibration scan, as well as the weights used when imaging the target.
Calibration pipelines should routinely employ complete field models for
standard calibrators to remove this potentially destructive contaminant from
the data, a recommendation we validate by comparing our simulation results to a
MeerKAT scan of PKS B1934-638, calibrated with and without our expanded sky
model.Comment: 11 pages, 10 figures, accepted for publication in MNRA
Noncommutative generalizations of theorems of Cohen and Kaplansky
This paper investigates situations where a property of a ring can be tested
on a set of "prime right ideals." Generalizing theorems of Cohen and Kaplansky,
we show that every right ideal of a ring is finitely generated (resp.
principal) iff every "prime right ideal" is finitely generated (resp.
principal), where the phrase "prime right ideal" can be interpreted in one of
many different ways. We also use our methods to show that other properties can
be tested on special sets of right ideals, such as the right artinian property
and various homological properties. Applying these methods, we prove the
following noncommutative generalization of a result of Kaplansky: a (left and
right) noetherian ring is a principal right ideal ring iff all of its maximal
right ideals are principal. A counterexample shows that the left noetherian
hypothesis cannot be dropped. Finally, we compare our results to earlier
generalizations of Cohen's and Kaplansky's theorems in the literature.Comment: 41 pages. To appear in Algebras and Representation Theory. Minor
changes were made to the numbering system, in order to remain consistent with
the published versio
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