Assembly and test of the gas pixel detector for X-ray polarimetry Li, H.; Feng, H.; Muleri, F. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
12/2015, Volume:
804
Journal Article
Peer reviewed
Open access
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 0.8atm, the measured energy resolution is 18% at 6keV 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 6keV. The residual modulation is found to be 0.30±0.15% at 6keV 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 80μm in FWHM, consistent with previous studies and sufficient for XIPE requirements.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The Imaging X-ray Polarimetry Explorer (IXPE) Sgrò, C.
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
08/2019, Volume:
936
Journal Article
Peer reviewed
Recently selected by NASA as the next SMall EXplorer (SMEX) mission for a launch in 2021, the Imaging X-ray Polarimetry Explorer (IXPE) marks the renaissance of X-ray polarimetry, with the prospect ...of measuring linear polarization from tens of astronomical X-ray sources. IXPE will expand the X-ray view of the Universe by providing new and unique insight into our understanding of some of the most violent celestial objects, such as stellar and supermassive black holes and neutron stars.
In this contribution we shall discuss the scientific prospects of the mission, as well as the status of the program, with particular emphasis on the development of the focal plane detectors sensitive to polarization.
•The Imaging X-ray Polarimetry Explorer (IXPE) will be the next NASA SMall EXplorer.•IXPE will be devoted to X-ray polarimetry in the 2–8 keV energy band.•The Gas Pixel Detector (GPD) made possible polarimetry at a few keV energy range.•The GPD exploit photoelectric effect in gas, with an ASIC as collecting anode.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
We examined latitudinal variation in adult and larval heat tolerance in Drosophila melanogaster from eastern Australia. Adults were assessed using static and ramping assays. Basal and hardened static ...heat knockdown time showed significant linear clines; heat tolerance increased towards the tropics, particularly for hardened flies, suggesting that tropical populations have a greater hardening response. A similar pattern was evident for ramping heat knockdown time at 0.06 °C min⁻¹ increase. There was no cline for ramping heat knockdown temperature (CTmax) at 0.1 °C min⁻¹ increase. Acute (static) heat knockdown temperature increased towards temperate latitudes, probably reflecting a greater capacity of temperate flies to withstand sudden temperature increases during summer in temperate Australia. Larval viability showed a quadratic association with latitude under heat stress. Thus, patterns of heat resistance depend on assay methods. Genetic correlations in thermotolerance across life stages and evolutionary potential for critical thermal limits should be the focus of future studies.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
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°. 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, 3.7, 4.0, 5.2 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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
X-rays are particularly suited to probing the physics of extreme objects. However, despite the enormous improvements of X-ray astronomy in imaging, spectroscopy, and timing, polarimetry remains ...largely unexplored. We propose the photoelectric polarimeter Gas Pixel Detector (GPD) as a candidate instrument to fill the gap created by more than 30 yr without measurements. The GPD, in the focus of a telescope, will increase the sensitivity of orders of magnitude. Moreover, since it can measure the energy, the position, the arrival time, and the polarization angle of every single photon, it allows us to perform polarimetry of subsets of data singled out from the spectrum, the light curve, or an image of the source. The GPD has an intrinsic, very fine imaging capability, and in this work we report on the calibration campaign carried out in 2012 at the PANTER X-ray testing facility of the Max-Planck-Institut fur extraterrestrische Physik of Garching (Germany) in which, for the first time, we coupled it with a JET-X optics module with a focal length of 3.5 m and an angular resolution of 18 arc sec at 4.5 keV. This configuration was proposed in 2012 aboard the X-ray Imaging Polarimetry Explorer (XIPE) in response to the ESA call for a small mission. We derived the imaging and polarimetric performance for extended sources like pulsar wind nebulae and supernova remnants as case studies for the XIPE configuration and also discuss possible improvements by coupling the detector with advanced optics that have a finer angular resolution and larger effective areas to study extended objects with more detail.
Latitudinal clines are considered a powerful means of investigating evolutionary responses to climatic selection in nature. However, most clinal studies of climatic adaptation in Drosophila have ...involved species that contain cosmopolitan inversion polymorphisms that show clinal patterns themselves, making it difficult to determine whether the traits or inversions are under selection. Further, although climatic selection is unlikely to act on only one life stage in metamorphic organisms, a few studies have examined clinal patterns across life stages. Finally, clinal patterns of heat tolerance may also depend on the assay used. To unravel these potentially confounding effects on clinal patterns of thermal tolerance, we examined adult and larval heat tolerance traits in populations of Drosophila simulans from eastern Australia using static and dynamic (ramping 0.06 °C min−1) assays. We also used microsatellites markers to clarify whether demographic factors or selection are responsible for population differentiation along clines. Significant cubic clinal patterns were observed for adult static basal, hardened and dynamic heat knockdown time and static basal heat survival in larvae. In contrast, static, hardened larval heat survival increased linearly with latitude whereas no clinal association was found for larval ramping survival. Significant associations between adult and larval traits and climatic variables, and low population differentiation at microsatellite loci, suggest a role for climatic selection, rather than demographic processes, in generating these clinal patterns. Our results suggest that adaptation to thermal stress may be species and life‐stage specific, complicating our efforts to understand the evolutionary responses to selection for increasing thermotolerance.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
ABSTRACT Most of the celestial γ rays detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope originate from the interstellar medium when energetic cosmic rays ...interact with interstellar nucleons and photons. Conventional point-source and extended-source studies rely on the modeling of this diffuse emission for accurate characterization. Here, we describe the development of the Galactic Interstellar Emission Model (GIEM), which is the standard adopted by the LAT Collaboration and is publicly available. This model is based on a linear combination of maps for interstellar gas column density in Galactocentric annuli and for the inverse-Compton emission produced in the Galaxy. In the GIEM, we also include large-scale structures like Loop I and the Fermi bubbles. The measured gas emissivity spectra confirm that the cosmic-ray proton density decreases with Galactocentric distance beyond 5 kpc from the Galactic Center. The measurements also suggest a softening of the proton spectrum with Galactocentric distance. We observe that the Fermi bubbles have boundaries with a shape similar to a catenary at latitudes below 20° and we observe an enhanced emission toward their base extending in the north and south Galactic directions and located within ∼4° of the Galactic Center.
The flexibility afforded to genotypes in different environments by phenotypic plasticity is of interest to biologists studying thermal adaptation because of the thermal lability of many traits. ...Differences in thermal performance and reaction norms can provide insight into the evolution of thermal adaptation to explore broader questions such as species distributions and persistence under climate change. One approach is to study the effects of temperature on fitness, morphological and more recently gene expression traits in populations from different climatic origins. The diverse climatic conditions experienced by Drosophila melanogaster along the eastern Australian temperate‐tropical gradient are ideal given the high degree of continuous trait differentiation, but reaction norm variation has not been well studied in this system. Here, we reared a tropical and temperate population from the ends of the gradient over six developmental temperatures and examined reaction norm variation for five quantitative traits including thermal performance for fecundity, and reaction norms for thermotolerance, body size, viability and 23 transcript‐level traits. Despite genetic variation for some quantitative traits, we found no differentiation between the populations for fecundity thermal optima and breadth, and the reaction norms for the other traits were largely parallel, supporting previous work suggesting that thermal evolution occurs by changes in trait means rather than by reaction norm shifts. We examined reaction norm variation in our expanded thermal regime for a gene set shown to previously exhibit GxE for expression plasticity in east Australian flies, as well as key heat‐shock genes. Although there were differences in curvature between the populations suggesting a higher degree of thermal plasticity in expression patterns than for the quantitative traits, we found little evidence to support a role for genetic variation in maintaining expression plasticity.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
We present our study on the reconstruction of photoelectron tracks in gas pixel detectors used for astrophysical X-ray polarimetry. Our work aims to maximize the performance of convolutional neural ...networks (CNNs) to predict the impact point of incoming X-rays from the image of the photoelectron track. A very high precision in the reconstruction of the impact point position is achieved thanks to the introduction of an artificial sharpening process of the images. We find that providing the CNN-predicted impact point as input to the state-of-the-art analytic analysis improves the modulation factor (~1% at 3 keV and ~6% at 6 keV) and naturally mitigates a subtle effect appearing in polarization measurements of bright extended sources known as “polarization leakage”.
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