160 research outputs found
RR-Lyrae-type pulsations from a 0.26-solar-mass star in a binary system
RR Lyrae pulsating stars have been extensively used as tracers of old stellar
populations for the purpose of determining the ages of galaxies, and as tools
to measure distances to nearby galaxies. There was accordingly considerable
interest when the RR Lyr star OGLE-BLG-RRLYR-02792 was found to be a member in
an eclipsing binary system4, as the mass of the pulsator (hitherto constrained
only by models) could be unambiguously determined. Here we report that
RRLYR-02792 has a mass of 0.26 M_sun and therefore cannot be a classical RR
Lyrae star. Through models we find that its properties are best explained by
the evolution of a close binary system that started with 1.4 M_sun and 0.8
M_sun stars orbiting each other with an initial period of 2.9 days. Mass
exchange over 5.4 Gyr produced the observed system, which is now in a very
short-lived phase where the physical properties of the pulsator happen to place
it in the same instability strip of the H-R diagram occupied by RR Lyrae stars.
We estimate that samples of RR Lyr stars may contain a 0.2 percent
contamination with systems similar to this one, implying that distances
measured with RR Lyrae stars should not be significantly affected by these
binary interlopers
Optimisation of the event-based TOF filtered back-projection for online imaging in total-body J-PET
We perform a parametric study of the newly developed time-of-flight (TOF)
image reconstruction algorithm, proposed for the real-time imaging in
total-body Jagiellonian PET (J-PET) scanners. The asymmetric 3D filtering
kernel is applied at each most likely position of electron-positron
annihilation, estimated from the emissions of back-to-back -photons.
The optimisation of its parameters is studied using Monte Carlo simulations of
a 1-mm spherical source, NEMA IEC and XCAT phantoms inside the ideal J-PET
scanner. The combination of high-pass filters which included the TOF filtered
back-projection (FBP), resulted in spatial resolution, 1.5 higher in
the axial direction than for the conventional 3D FBP. For realistic -minute
scans of NEMA IEC and XCAT, which require a trade-off between the noise and
spatial resolution, the need for Gaussian TOF kernel components, coupled with
median post-filtering, is demonstrated. The best sets of 3D filter parameters
were obtained by the Nelder-Mead minimisation of the mean squared error between
the resulting and reference images. The approach allows training the
reconstruction algorithm for custom scans, using the IEC phantom, when the
temporal resolution is below 50 ps. The image quality parameters, estimated for
the best outcomes, were systematically better than for the non-TOF FBP
Testing CPT symmetry in ortho-positronium decays with positronium annihilation tomography
Charged lepton system symmetry under combined charge, parity, and time-reversal transformation (CPT) remains scarcely tested. Despite stringent quantum-electrodynamic limits, discrepancies in predictions for the electron–positron bound state (positronium atom) motivate further investigation, including fundamental symmetry tests. While CPT noninvariance effects could be manifested in non-vanishing angular correlations between final-state photons and spin of annihilating positronium, measurements were previously limited by knowledge of the latter. Here, we demonstrate tomographic reconstruction techniques applied to three-photon annihilations of ortho-positronium atoms to estimate their spin polarisation without magnetic field or polarised positronium source. We use a plastic-scintillator-based positron-emission-tomography scanner to record ortho-positronium (o-Ps) annihilations with single-event estimation of o-Ps spin and determine the complete spectrum of an angular correlation operator sensitive to CPT-violating effects. We find no violation at the precision level of 10−4, with an over threefold improvement on the previous measurement
Estimating relationship between the time over threshold and energy loss by photons in plastic scintillators used in the J-PET scanner
From tests of discrete symmetries to medical imaging with J-PET detector
We present results on CPT symmetry tests in decays of positronium performed with the precision at the level of 10, and positronium images determined with the prototype of the J-PET tomograph. The first full-scale prototype apparatus consists of 192 plastic scintillator strips readout from both ends with vacuum tube photomultipliers. Signals produced by photomultipliers are probed in the amplitude domain and are digitized by FPGA-based readout boards in triggerless mode. In this contribution we report on the first two- and three-photon positronium images and tests of CPT symmetry in positronium decays
The influence of the landscape structure within buffer zones, catchment land use and instream environmental variables on mollusc communities in a medium-sized lowland river
The world’s freshwater molluscan fauna is facing unprecedented threats from habitat loss and degradation. Declines in native populations are mostly attributed to the human impact, which results in reduced water quality. The objectives of our survey were to analyse the structure of the mollusc communities in a medium-sized lowland river and to determine the most important environmental variables at different spatial scales, including landscape structure, catchment land use and instream environmental factors that influence their structure. Our survey showed that a medium-sized river, that flows through areas included in the European Ecological Natura 2000 Network Programme of protected sites, provides diverse instream habitats and niches that support 47 mollusc species including Unio crassus, a bivalve of Community interest, whose conservation requires the designation of a special conservation area under the Habitats Directive Natura 2000. This survey showed that mollusc communities are impacted by several environmental variables that act together at multiple scales. The landscape structure within buffer zones, catchment land use and instream environmental variables were all important and influenced the structure of mollusc communities. Therefore, they should all be taken into consideration in the future restoration of the river, future management projects and programmes for the conservation of biodiversity in running waters. The results of this study may be directly applicable for the rehabilitation of river ecosystems and are recommended to stakeholders in their future decision concerning landscape planning, monitoring species and their habitats, conservation plans and management in accordance with the requirements of sustainable development
Кераміка «terra sigillata» з с. Зимне на Волині
Стаття присвячена публікації чотирьох керамічних посудин типу «terra sigillata», знайдених на дні р. Луги у с. Зимне Володимир-Волинського району Волинської області. Попередній аналіз цих знахідок дозволяє віднести їх до Понтійського центру виробництва такого посуду. Вірогідним шляхом потрапляння цієї колекції на Волинь була готська експансія у Північне Причорномор’я
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