4,186 research outputs found
Baseline design of the filters for the LAD detector on board LOFT
The Large Observatory for X-ray Timing (LOFT) was one of the M3 missions
selected for the phase A study in the ESA's Cosmic Vision program. LOFT is
designed to perform high-time-resolution X-ray observations of black holes and
neutron stars. The main instrument on the LOFT payload is the Large Area
Detector (LAD), a collimated experiment with a nominal effective area of ~10 m
2 @ 8 keV, and a spectral resolution of ~240 eV in the energy band 2-30 keV.
These performances are achieved covering a large collecting area with more than
2000 large-area Silicon Drift Detectors (SDDs) each one coupled to a collimator
based on lead-glass micro-channel plates. In order to reduce the thermal load
onto the detectors, which are open to Sky, and to protect them from out of band
radiation, optical-thermal filter will be mounted in front of the SDDs.
Different options have been considered for the LAD filters for best compromise
between high quantum efficiency and high mechanical robustness. We present the
baseline design of the optical-thermal filters, show the nominal performances,
and present preliminary test results performed during the phase A study.Comment: Proc. SPIE 9144, Space Telescopes and Instrumentation 2014:
Ultraviolet to Gamma Ray, 91446
Social welfare in one-sided matchings: Random priority and beyond
We study the problem of approximate social welfare maximization (without
money) in one-sided matching problems when agents have unrestricted cardinal
preferences over a finite set of items. Random priority is a very well-known
truthful-in-expectation mechanism for the problem. We prove that the
approximation ratio of random priority is Theta(n^{-1/2}) while no
truthful-in-expectation mechanism can achieve an approximation ratio better
than O(n^{-1/2}), where n is the number of agents and items. Furthermore, we
prove that the approximation ratio of all ordinal (not necessarily
truthful-in-expectation) mechanisms is upper bounded by O(n^{-1/2}), indicating
that random priority is asymptotically the best truthful-in-expectation
mechanism and the best ordinal mechanism for the problem.Comment: 13 page
Terminal investment induced by a bacteriophage in a rhizosphere bacterium.
Despite knowledge about microbial responses to abiotic stress, few studies have investigated stress responses to antagonistic species, such as competitors, predators and pathogens. While it is often assumed that interacting populations of bacteria and phage will coevolve resistance and exploitation strategies, an alternative is that individual bacteria tolerate or evade phage predation through inducible responses to phage presence. Using the microbial model Pseudomonas fluorescens SBW25 and its lytic DNA phage SBW25Φ2, we demonstrate the existence of an inducible response in the form of a transient increase in population growth rate, and found that the response was induced by phage binding. This response was accompanied by a decrease in bacterial cell size, which we propose to be an associated cost. We discuss these results in the context of bacterial ecology and phage-bacteria co-evolution
Discovery of a Peculiar Dip from GX 301-2
We present temporal and spectral properties of a unique X-ray dip in GX 301-2
as seen with Rossi X-ray Timing Explorer in May 2010. The X-ray pulsation from
the source gradually declined prior to the dip, disappears for one spin cycle
during the dip and is abruptly restored in the spin cycle immediately after the
dip. Moreover, the phase-integrated spectrum of the source becomes softer
before and during the dip and it quickly hardens again following the dip. Our
findings indicate the fact that the mechanism for pulsations gradually turned
off briefly and underlying dim and softer emission likely from the accretion
column became observable in the brief absence of high level emission due to
wind accretion.Comment: Accepted for publication in A&A Letter
Calibration and monitoring of the ASTRI-Horn telescope by using the night-sky background measured by the photon-statistics ("variance") method
ASTRI-Horn is the Cherenkov telescope developed by INAF and operating in
Italy on the slopes of Etna volcano. Characterized by a dual-mirror optical
system and a focal plane covered by silicon photomultiplier sensors, ASTRI-Horn
is a prototype of the telescopes proposed to form one of the pathfinder
sub-arrays of the Cherenkov Telescope Array Observatory in Chile. The
electronics of the ASTRI-Horn telescope, optimized to detect nanosecond burst
of light, is not able to directly measure any continuous or slowly varying flux
illuminating its camera. To measure the intensity of the night sky background
(NSB) in the field of view of the telescope, the firmware of the ASTRI-Horn
camera continuously performs the statistical analysis of its detector signals
and periodically provides in output the "variance" of each pixel, which is
linearly dependent on the rate of detected photons; in this way, an indirect,
but accurate measurement of the NSB flux is obtained without interference with
the normal telescope operation. In this contribution we provide an overview of
several calibration and monitoring tasks that can be performed in a
straightforward way by the analysis of the "variance" data such as the camera
astrometry, the actual telescope orientation and the monitoring of its optical
point spread function
The 0.1-100 keV Spectrum of LMC X-4 in the High State: Evidence for a High Energy Cyclotron Absorption Line
We report on the spectral analysis of the X-ray pulsar LMC X-4 in its high
state out of eclipse observed by BeppoSAX. During this observation no coherent
pulsations are detected. The primary continuum is well described by a power law
with a high energy cutoff (E_cutoff ~ E_fold ~ 18 keV). The addition of a
cyclotron absorption line at ~100 keV improves the fit significantly. The
inferred magnetic moment is 1.1 10^{31} Gauss cm^3, in agreement with the value
estimated assuming that the neutron star is at the spin equilibrium, as it has
been proposed for this source. The remaining excess at low energies can be
fitted by a Comptonization of soft photons by moderately hot electrons (kT ~0.9
keV), with an optical depth \tau ~ 16. The seed photons for this Comptonization
are consistent with black body emission from the accretion disk at the
magnetospheric radius. Another possibility is to fit the soft excess with black
body and thermal bremsstrahlung. In this case the black body would originate
from cold plasma at the magnetosphere while the bremsstrahlung component may be
produced by the strong stellar wind from the companion star, ionized by the
X-ray emission from the pulsar.Comment: 13 pages and 6 figures. Apj, in pres
The 0.1-100 keV Spectrum of Centaurus X-3: Pulse Phase Spectroscopy of the Cyclotron Line and Magnetic Field Structure
We report spectral and temporal analysis of the X-ray pulsar Centaurus X-3 out of eclipse observed by BeppoSAX. The broadband spectrum (0.12-100 keV) is well described by an absorbed power law modified by a high-energy rollover at ~14 keV (e-folding energy ~8 keV) plus an iron emission line at ~6.7 keV. A soft excess below 1 keV is also present. Interpreted as a blackbody (kT 0.1 keV), it corresponds to 58% of the total unabsorbed flux. This component seems to originate from reprocessing of the primary radiation by an opaque shell located at the magnetosphere. An absorption feature at ~30 keV is also present. Interpreted as a cyclotron line, after correction for gravitational redshift, this corresponds to a surface magnetic field of ~3.5 × 1012 G. Phase-resolved spectroscopy reveals a variation by about 8 keV of the cyclotron resonance energy along the pulse profile. In particular, the line energy decreases from ~36 to ~28 keV along the peak, starting from the ascent. The asymmetric variations of the cyclotron line energy can be explained by assuming an offset of the dipolar magnetic field with respect to the neutron star center. The spectral results are discussed in terms of emission from magnetic caps, where Comptonization of soft photons occurs. The soft photons could come from either magnetically resonant double Compton scattering or from illumination of the polar cap by the primary radiation reprocessed at the magnetospheric surface, a feedback mechanism similar to that proposed for the formation of black hole spectra
The homeobox gene Hex is required in definitive endodermal tissues for normal forebrain, liver and thyroid formation
The homeobox gene Hex is expressed in the anterior visceral endoderm (AVE) and rostral definitive endoderm of early mouse embryos. Later, Hex transcripts are detected in liver, thyroid and endothelial precursor cells. A null mutation was introduced into the Hex locus by homologous recombination in embryonic stem cells. Hex mutant embryos exhibit varying degrees of anterior truncation as well as liver and thyroid dysplasia. The liver diverticulum is formed but migration of hepatocytes into the septum transversum fails to occur. Development of the thyroid is arrested at the thyroid bud stage at 9.5 dpc. Brain defects are restricted to the rostral forebrain and have a caudal limit at the zona limitans intrathalamica, the boundary between dorsal and ventral thalamus. Analysis of Hex(−/−) mutants at early stages shows that the prospective forebrain ectoderm is correctly induced and patterned at 7.5 days post coitum (dpc), but subsequently fails to develop. AVE markers are expressed and correctly positioned but development of rostral definitive endoderm is greatly disturbed in Hex(−/−) embryos. Chimeric embryos composed of Hex(−/−) cells developing within a wild-type visceral endoderm show forebrain defects indicating that Hex is required in the definitive endoderm. All together, these results demonstrate that Hex function is essential in definitive endoderm for normal development of the forebrain, liver and thyroid gland
Age, Metallicity and Star Formation History of Cluster Galaxies at z~0.3 F
We investigate the color-magnitude distribution in the rich cluster AC 118 at
z=0.31. The sample is selected by the photometric redshift technique, allowing
to study a wide range of properties of stellar populations, and is complete in
the K-band, allowing to study these properties up to a given galaxy mass. We
use galaxy templates based on population synthesis models to translate the
physical properties of the stellar populations - formation epoch, time-scale of
star formation, and metallicity - into observed magnitudes and colors. In this
way we show that a sharp luminosity-metallicity relation is inferred without
any assumption on the galaxy formation scenario (either monolithic or
hierarchical). Our data exclude significant differences in star formation
histories along the color-magnitude relation, and therefore confirm a pure
metallicity interpretation for its origin, with an early (z~5) formation epoch
for the bulk of stellar populations. The dispersion in the color-magnitude
diagram implies that fainter galaxies in our sample (K~18) ceased to form stars
as late as z~0.5, in agreement with the picture that these galaxies were
recently accreted into the cluster environment. The trend with redshift of the
total stellar mass shows that half of the luminous mass in AC 118 was already
formed at $z~2, but also that 20% of the stars formed at z<1.Comment: 16 pages, 10 figures. ApJ in pres
Enrichment-Planting with Pines Alters Fuel Amount and Structure in Endangered Araucaria Araucana Forests in Northwestern Patagonia, Argentina
The introduction of non-native tree species for large-scale afforestation may alter the fire regime of native ecosystems by modifying fuel proprieties. We quantified changes in fuel abundance and structure resulting from the establishment of commercial Pinus spp. plantations in Araucaria araucana ecosystems in northwestern Patagonia, Argentina. Specifically, we assessed the amount, distribution, and condition (live/dead) of surface and standing fine fuel in A. araucana stands with mature pine plantations (i.e. \u3e 20 cm dbh) and in stands dominated only by A. araucana (control). Our study shows that both types of stands are prone to wildfires, but pine plantations have fuel characteristics that imply greater flammability due to higher fuel load and vertical continuity in the understory and in the overstory canopy. In the absence of fuel mitigation practices, A. araucana stands with plantations exhibit greater flammability than the control A. araucana stands, potentially promoting the occurrence and spread of fires of greater severity. This study contributes to understanding the effects of enrichment planting of pines, and possibly pine invasions, on A. araucaria ecosystem flammability and their potential consequences on fire behavior
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