50 research outputs found
Spectral and polarimetric characterization of the Gas Pixel Detector filled with dimethyl ether
The Gas Pixel Detector belongs to the very limited class of gas detectors
optimized for the measurement of X-ray polarization in the emission of
astrophysical sources. The choice of the mixture in which X-ray photons are
absorbed and photoelectrons propagate, deeply affects both the energy range of
the instrument and its performance in terms of gain, track dimension and
ultimately, polarimetric sensitivity. Here we present the characterization of
the Gas Pixel Detector with a 1 cm thick cell filled with dimethyl ether (DME)
at 0.79 atm, selected among other mixtures for the very low diffusion
coefficient. Almost completely polarized and monochromatic photons were
produced at the calibration facility built at INAF/IASF-Rome exploiting Bragg
diffraction at nearly 45 degrees. For the first time ever, we measured the
modulation factor and the spectral capabilities of the instrument at energies
as low as 2.0 keV, but also at 2.6 keV, 3.7 keV, 4.0 keV, 5.2 keV and 7.8 keV.
These measurements cover almost completely the energy range of the instrument
and allows to compare the sensitivity achieved with that of the standard
mixture, composed of helium and DME.Comment: 20 pages, 11 figures, 5 tables. Accepted for publication by NIM
Low energy polarization sensitivity of the Gas Pixel Detector
An X-ray photoelectric polarimeter based on the Gas Pixel Detector has been
proposed to be included in many upcoming space missions to fill the gap of
about 30 years from the first (and to date only) positive measurement of
polarized X-ray emission from an astrophysical source. The estimated
sensitivity of the current prototype peaks at an energy of about 3 keV, but the
lack of readily available polarized sources in this energy range has prevented
the measurement of detector polarimetric performances.
In this paper we present the measurement of the Gas Pixel Detector
polarimetric sensitivity at energies of a few keV and the new, light, compact
and transportable polarized source that was devised and built to this aim.
Polarized photons are produced, from unpolarized radiation generated with an
X-ray tube, by means of Bragg diffraction at nearly 45 degrees.
The employment of mosaic graphite and flat aluminum crystals allow the
production of nearly completely polarized photons at 2.6, 3.7 and 5.2 keV from
the diffraction of unpolarized continuum or line emission. The measured
modulation factor of the Gas Pixel Detector at these energies is in good
agreement with the estimates derived from a Monte Carlo software, which was up
to now employed for driving the development of the instrument and for
estimating its low energy sensitivity. In this paper we present the excellent
polarimetric performance of the Gas Pixel Detector at energies where the peak
sensitivity is expected. These measurements not only support our previous
claims of high sensitivity but confirm the feasibility of astrophysical X-ray
photoelectric polarimetry.Comment: 15 pages, 12 figures. Accepted for publication in NIM
Assembly and test of the gas pixel detector for X-ray polarimetry
The gas pixel detector (GPD) dedicated for photoelectric X-ray polarimetry is selected as the focal plane detector for the ESA medium-class mission concept X-ray Imaging and Polarimetry Explorer (XIPE). Here we show the design, assembly, and preliminary test results of a small GPD for the purpose of gas mixture optimization needed for the phase A study of XIPE. The detector is assembled in house at Tsinghua University following a design by the INFN-Pisa group. The improved detector design results in a good uniformity for the electric field. Filled with pure dimethyl ether (DME) at 0.8 atm, the measured energy resolution is 18% at 6 keV and inversely scales with the square root of the X-ray energy. The measured modulation factor is well consistent with that from simulation, up to ~0.6 above 6 keV. The residual modulation is found to be 0.30±0.15% at 6 keV for the whole sensitive area, which can be translated into a systematic error of less than 1% for polarization measurement at a confidence level of 99%. The position resolution of the detector is about 80 μm in FWHM, consistent with previous studies and sufficient for XIPE requirements
The VVV near-IR galaxy catalogue in a Northern part of the Galactic disc
The automated identification of extragalactic objects in large surveys
provides reliable and reproducible samples of galaxies in less time than
procedures involving human interaction. However, regions near the Galactic disc
are more challenging due to the dust extinction. We present the methodology for
the automatic classification of galaxies and non-galaxies at low Galactic
latitude regions using both images and, photometric and morphological near-IR
data from the VVVX survey. Using the VVV-NIRGC, we analyse by statistical
methods the most relevant features for galaxy identification. This catalogue
was used to train a CNN with image data and an XGBoost model with both
photometric and morphological data and then to generate a dataset of
extragalactic candidates. This allows us to derive probability catalogues used
to analyse the completeness and purity as a function of the configuration
parameters and to explore the best combinations of the models. As a test case,
we apply this methodology to the Northern disc region of the VVVX survey,
obtaining 172,396 extragalatic candidates with probabilities of being galaxies.
We analyse the performance of our methodology in the VVV disc, reaching an
F1-score of 0.67, a 65 per cent purity and a 69 per cent completeness. We
present the VVV-NIR Galaxy Catalogue: Northern part of the Galactic disc
comprising 1,003 new galaxies, with probabilities greater than 0.6 for either
model, with visual inspection and with only 2 previously identified galaxies.
In the future, we intend to apply this methodology to other areas of the VVVX
survey.Comment: 12 pages, 14 figures, accepted in MNRA
Imaging with the invisible light
We describe a UV photo-detector with single photon(electron) counting and
imaging capability. It is based on a CsI photocathode, a GEM charge multiplier
and a self triggering CMOS analog pixel chip with 105k pixels at 50 micron
pitch. The single photoelectron produced by the absorption of a UV photon is
drifted to and multiplied inside a single GEM hole. The coordinates of the GEM
avalanche are reconstructed with high accuracy (4 micron rms) by the pixel
chip. As a result the map of the GEM holes, arranged on a triangular pattern at
50micron pitch, is finely imaged.Comment: 6 pages, 14 figures, presented at the 11th Vienna Conference on
Instrumentation VIC 2007, submitted to Nuclear Instruments and Methods
El activismo religioso conservador en Latinoamérica
Pertenece a la colección Religión, Género y Sexualidad / dirigida por Juan M. Vaggione ; v.3El libro reúne un conjunto de análisis que ponen en evidencia la heterogeneidad y la complejidad de las estrategias de influencia de la religión como uno de los principales sostenes de sistemas heteronormativos en nuestras sociedades contemporáneas. Tanto al nivel de los actores como de los discursos, las formas en que las religiones influencian las discusiones sobre políticas de sexualidad han adoptado diversas modalidades de actuación adaptándose a los momentos políticos. Así, los artículos que integran la compilación se dirigen a complejizar el rol de lo religioso en la política sexual contemporánea incluyendo en esta problematización distintos ejes de análisis y marcos teóricos. Haciendo foco en diferentes países de América Latina, con sus escenarios y problemas, los artículos abordan las variadas dimensiones del activismo religioso conservador en nuestra región
Fermi LAT Search for Photon Lines from 30 to 200 GeV and Dark Matter Implications
Dark matter (DM) particle annihilation or decay can produce monochromatic
-rays readily distinguishable from astrophysical sources. -ray
line limits from 30 GeV to 200 GeV obtained from 11 months of Fermi Large Area
Space Telescope data from 20-300 GeV are presented using a selection based on
requirements for a -ray line analysis, and integrated over most of the
sky. We obtain -ray line flux upper limits in the range , and give corresponding DM annihilation
cross-section and decay lifetime limits. Theoretical implications are briefly
discussed.Comment: 6 pages, 1 figure. Accepted for publication by The Physical Review
Letter
Search for extended gamma-ray emission from the Virgo galaxy cluster with Fermi-LAT
Galaxy clusters are one of the prime sites to search for dark matter (DM)
annihilation signals. Depending on the substructure of the DM halo of a galaxy
cluster and the cross sections for DM annihilation channels, these signals
might be detectable by the latest generation of -ray telescopes. Here
we use three years of Fermi Large Area Telescope (LAT) data, which are the most
suitable for searching for very extended emission in the vicinity of nearby
Virgo galaxy cluster. Our analysis reveals statistically significant extended
emission which can be well characterized by a uniformly emitting disk profile
with a radius of 3\deg that moreover is offset from the cluster center. We
demonstrate that the significance of this extended emission strongly depends on
the adopted interstellar emission model (IEM) and is most likely an artifact of
our incomplete description of the IEM in this region. We also search for and
find new point source candidates in the region. We then derive conservative
upper limits on the velocity-averaged DM pair annihilation cross section from
Virgo. We take into account the potential -ray flux enhancement due to
DM sub-halos and its complex morphology as a merging cluster. For DM
annihilating into , assuming a conservative sub-halo model
setup, we find limits that are between 1 and 1.5 orders of magnitude above the
expectation from the thermal cross section for
. In a more optimistic scenario, we
exclude
for for the same channel. Finally, we
derive upper limits on the -ray-flux produced by hadronic cosmic-ray
interactions in the inter cluster medium. We find that the volume-averaged
cosmic-ray-to-thermal pressure ratio is less than .Comment: 15 pages, 11 figures, 4 tables, accepted for publication in ApJ;
corresponding authors: T. Jogler, S. Zimmer & A. Pinzk
The influence of superstructures on bright galaxy environments: clustering properties
We analyse the dependence of clustering properties of galaxies as a function
of their large-scale environment. In order to characterize the environment on
large scales, we use the catalogue of future virialized superstructures (FVS)
by Luparello et al. and separate samples of luminous galaxies according to
whether or not they belong to FVS. In order to avoid biases in the selection of
galaxies, we have constructed different subsamples so that the distributions of
luminosities and masses are comparable outside and within FVS. As expected, at
large scales, there is a strong difference between the clustering of galaxies
inside and outside FVS. However, this behaviour changes at scales r < 1 Mpc/h,
where the correlations have similar amplitudes. The amplitude of the two-halo
term of the correlation function for objects inside FVS does not depend on
their mass, but rather on that of the FVS. This is confirmed by comparing this
amplitude with that expected from extended Press-Schechter fits. In order to
compare these observational results with current models for structure
formation, we have performed a similar analysis using a semi-analytic
implementation in a LCDM cosmological model. We find that the cross-correlation
functions from the mock catalogue depend on the large-scale structures in a
similar way to the observations. From our analysis, we conclude that the
clustering of galaxies within the typical virialized regions of groups, mainly
depends on the halo mass, irrespective of the large-scale environment.Comment: 12 pages, 9 figures, modified to match accepted version in MNRA
Interpopulation variation in female remating is attributable to female and male effects in Callosobruchus chinensis
The evolution of female multiple mating is best understood by consideration of male and female reproductive perspectives. Females should usually be selected to remate at their optimum frequencies whereas males should be selected to manipulate female remating to their advantage. Female remating behavior may therefore be changed by variation of male and female traits. In this study, our objective was to separate the effects of female and male strains on female remating for the adzuki bean beetle, Callosobruchus chinensis, for which there is interstrain variation in female remating frequency. We found that interstrain variation in female remating is primarily attributable to female traits, suggesting genetic variation in female receptivity to remating in C. chinensis. Some interstrain variation in female remating propensity was attributable to an interaction between female and male strains, however, with the males of some strains being good at inducing nonreceptivity in females from one high-remating strain whereas others were good at inducing copulation in nonvirgin females from the high-remating strain. There is, therefore, interstrain variation in male ability to deter females from remating and in male ability to mate successfully with nonvirgin females. These results suggest that mating traits have evolved along different trajectories in different strains of C. chinensis.</p