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The polarisation of astrophysical source emission in the energy range from a few tens of keV up to the MeV region is an almost
unexplored field of high-energy astrophysics. Till date, polarimetry in astrophysics–in the energy domain from hard X-rays
up to soft γ-rays–has not been pursued due to the difficulties involved in obtaining sufficient sens...
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... the GEANT4 library modules, it was possible to construct an approximate model of the CIPHER position-sensitive detector, as well as the polarised beam characteristics that irradiates each detection plane module as shown in Figure 1. In Table I, the geometrical dimensions and material characteristics of the various parts of the detector are indicated as they were implemented in the Monte Carlo simulation code. Each pixel is labelled by an identification number ( x pixel ID , y pixel ID ) in the x – y plane, containing the detector surface. The irradiation of the detector is simulated by a 100% linearly polarised beam along the z -axis uniformly covering the four central pixels: (15, 15), (15, 16), (16, 15) and (16, 16). Then for each of the surrounding pixels, we obtain the number of double and higher order events, as well as the respective energy deposition. The performance of a scattering polarimeter instrument can be evaluated by analysing the distribution of double events through the polarimetric modulation factor, Q . This is obtained by integrating the Compton polarimetric differential cross section formula over the solid angles defined by the physical geometry of the detection plane and can be calculated by means ...
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... MeV the integrated modulation factor varies between 0.25 and 0.16, between 0°and 90°. The calculated Q obtained for each band can be explained by previous simulation and experimental work [16,17]. Actually, the modulation factor peaks for Compton photon scattering angles close to 90°(in the band Between~200 and~400 keV). ...
... Then it decreases dramatically when the average scattering angles becomes increasingly lower for higher source photons' energies. The observed modulation degradation with the incidence angle is also in agreement with previous simulation and experimental studies [16][17][18][19]. In fact, an off-axis beam generates a double-event distribution in the neighboring pixels which reflects not only the polarization status of the beam but as well its angular offset with respect to the normal incidence direction. ...
... In fact, an off-axis beam generates a double-event distribution in the neighboring pixels which reflects not only the polarization status of the beam but as well its angular offset with respect to the normal incidence direction. Depending on the beam linear polarization angle, it combines with the incidence angle to amplify or to smooth the effects generated in the modulation of double-events as show in [16] simulation analysis and in [17] ...
High-energy astrophysics polarimetry may significantly benefit from e-ASTROGAM and from AMEGO mission proposals, since to date limited polarimetric measurements were performed in this domain, exclusively under 1 MeV. The polarimetric potential of both missions was analyzed in the Compton regime by Monte Carlo mass model simulations with MEGAlib toolkit. The performance of e-ASTROGAM was analyzed by simulating Si tracker and calorimeter alternative configurations and detection materials, within missions’ volume, mass and power margins. The modulation polarimetric factor, Q, and the MDP were calculated for different polarized source types and for variable incidence angle measurement conditions. Finally, the polarimetric performances of both instruments was compared and analyzed. Q modulation factors obtained ranged between ~0.2 and ~0.4 in the 0.2–2.0 MeV band and MDP ~0.65% was estimated for a 100% polarized Crab type source, 1 Ms. observation time and 3σ significance.
... These prototypes were semiconductor based (mostly CdTe family) detection planes designed for coded mask or for Laue lens instrument solutions. These solutions require a trade-off between the imaging, spectroscopy and polarimetry components [24,25] that depends on the mission scientific objectives and which results in a substantially different configuration than those required for dedicated polarimeters [7,14,25]. So far there has been insufficient interest from the space agencies (including ESA) of our countries to accept a dedicated Compton polarimeter. ...
... There are two methods to deal with simultaneous multi-pixel irradiation: a statistical method and a hit order reconstruction method. A statistical method that takes into account the π symmetry of polarized Compton photon angular distribution can be easily implemented when energy and order of the hits are not relevant [24]. Herein, we adopt the reconstruction method that allows determining the order of the hits in each double event. ...
... In view of the power and the lack of precise polarization studies of GRBs, several X-and γ-ray polarimeters have been proposed and are under development. Some examples are POLAR [20], GRAPE (Gamma-Ray Polarimeter Experiment, [21]) POET (Polarimeters for Energetic Transients, [22]), CIPHER (Coded Imager and Polarimeter for High Energy Radiation, [23]), PHENEX (Polarimetry for High ENErgy X rays, [24]), XPOL [25] and POLARIX [26]. In addition, polarimeters designed for studying fixed sources, like PoGOLite [27], could also measure GRB if they happen to appear in their field of view. ...
The hard X-ray polarimeter POLAR aims to measure the linear polarization of the 50-500 keV photons arriving from the prompt emission of gamma-ray bursts (GRBs). The position in the sky of the detected GRBs is needed to determine their level of polarization. We present here a method by which, despite of the polarimeter incapability of taking images, GRBs can be roughly localized using POLAR alone. For this purpose scalers are attached to the output of the 25 multi-anode photomultipliers (MAPMs) that collect the light from the POLAR scintillator target. Each scaler measures how many GRB photons produce at least one energy deposition above 50 keV in the corresponding MAPM. Simulations show that the relative outputs of the 25 scalers depend on the GRB position. A database of very strong GRBs simulated at 10201 positions has been produced. When a GRB is detected, its location is calculated searching the minimum of the chi2 obtained in the comparison between the measured scaler pattern and the database. This GRB localization technique brings enough accuracy so that the error transmitted to the 100% modulation factor is kept below 10% for GRBs with fluence Ftot \geq 10^(-5) erg cm^(-2) . The POLAR localization capability will be useful for those cases where no other instruments are simultaneously observing the same field of view. Comment: 13 pages, 10 figures
... In fact the impinging photon polarisation status introduces a typical asymmetry of the double events (photons that first undergo a Compton interaction and are then absorbed through a second interaction with the material) distribution [78]. Therefore each segmented detector can be used in principle as a scatter polarimeter, but in particular CdZnTe/CdTe pixel detectors are very suitable for this type of measurements at high energy (> 100 keV) due to the fine segmentation achievable [79]. These detectors, due to their intrinsic material properties and geometric shape promise efficient polarimetric measurements between 100 keV and 1 MeV. ...
Over the last decade, cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) wide band gap semiconductors have attracted increasing interest as X-ray and gamma ray detectors. Among the traditional high performance spectrometers based on silicon (Si) and germanium (Ge), CdTe and CdZnTe detectors show high detection efficiency and good room temperature performance and are well suited for the development of compact and reliable detection systems. In this paper, we review the current status of research in the development of CdTe and CdZnTe detectors by a comprehensive survey on the material properties, the device characteristics, the different techniques for improving the overall detector performance and some major applications. Astrophysical and medical applications are discussed, pointing out the ongoing Italian research activities on the development of these detectors.
... In view of the power and the lack of precise polarization studies of GRBs, several X-and γ-ray polarimeters have been proposed and are under development. Some examples are POLAR [5], GRAPE (Gamma-Ray Polarimeter Experiment [6]), POET (Polarimeters for Energetic Transients, [7]), CIPHER (Coded Imager and Polarimeter for High Energy Radiation, [8]), PHENEX (Polarimetry for High ENErgy X rays, [9]), XPOL [10] and POLARIX [11]. ...
POLAR is a compact instrument devoted to measure the linear polarization of the gamma-ray bursts (GRBs) prompt emission. The position of the detected GRB with respect to the POLAR coordinates is needed to determine its level of po-larization. Using dedicated GEANT4 simulations we have developed a method by which, despite of the polarimeter incapability of taking images, GRBs can be roughly localized using POLAR alone. For this purpose scalers are attached to the output of the 25 multi-anode photomultipliers (MAPMs) that collect the light from the POLAR scintillator target. Simulations show that the relative outputs of the 25 scalers depend on the GRB position. A database of very strong GRBs simulated at 10201 positions has been produced. When a GRB is detected, its location is calculated searching the minimum of the χ 2 obtained in the comparison between the measured scaler pattern and the database. This GRB localization technique brings enough accuracy so that the error transmitted to the 100% modulation factor is kept below 10% for GRBs with fluence F tot ≥ 10 −5 erg cm −2 . The POLAR localization capability will be useful for those cases where no other instruments are simultaneously observing the same field of view.
... Instrument designs that emphasize the lower end of the energy band divide the active area between low-Z active scattering elements, such as plastic scintillator, and high-Z photon absorbers [14]. At the higher end of this energy range, homogeneous higher-Z detectors such as silicon, germanium, or CdTe are a viable means to large effective areas [15]. The high spatial resolution of TPCs is not an advantage in this energy band since astronomical imaging is best performed with coded apertures or collimators. ...
High-energy astrophysics has yet to exploit the unique and important information that polarimetry could provide, largely due to the limited sensitivity of previously available polarimeters. In recent years, numerous efforts have been initiated to develop instruments with the sensitivity required for astronomical polarimetry over the 100 eV to 10 GeV band. Time projection chambers (TPCs), with their high-resolution event imaging capability, are an integral part of some of these efforts. After a brief overview of current astronomical polarimeter development efforts, the role of TPCs will be described in more detail. These include TPCs as photoelectric X-ray polarimeters and TPCs as components of polarizationsensitive Compton and pair-production telescopes.
... The main advantage of this kind of detector is that high-sensitivity polarimetric measurements can be performed simultaneously with spectral imaging and time variability observations. The collaboration has already proposed an instrument concept to measure the Crab pulsar polarization level in the range between 50-1000 keV: the CIPHER (Coded Imager and Polarimeter for High Energy Radiation) telescope [Curado et al., 2003]: a Minimum Detectable Polarisation (MDP at 3σ) level below 10% is expected for a typical balloon observation time (10 4 s). Because of the scientific importance of polarimetry measurements it is the right strategy to implement this capability in the principal on-board instrumentation in the design of a Gamma-Ray Imager based on new hard-X and soft gamma ray focusing optics for the next ESA Cosmic Vision call for proposals (Cosmic Vision 2015-2025), it is strategic to implement this capability in the principal on-board instrumentation. ...
... The collaboration between the two research groups has already given rise to both extensive Monte Carlo (MC) studies [Curado et al., 2003] and experimental tests of pixel CdTe matrices used as polarimeters that have confirmed the reliability of our MC codes. These Monte Carlo simulations allowed both the study of CdTe polarimeters (Q factor and detection efficiency) and the study of several problems and effects associated with the interaction of polarized radiation in planar pixel CdTe detectors, such as the influence on polarimetric performance of the X-and γ-ray incidence inclination with respect to the optical axis, full irradiated detector surface analysis techniques and the variation of modulation factor as a function of the angle between the polarisation direction and the detection plane axis. ...
... The mean Compton scattering angle decreases as the photon energy increases and therefore the number of double events occurring at polar angles around 90º is lower. As shown in previous Monte Carlo simulation studies performed over a 100 keV to 1 MeV energy range this effect becomes important for photon energies higher then 400 keV [Curado et al., 2003]. From Table I it is evident that the double event absolute efficiency obtained is higher for thicker focal planes under the same simulation conditions and it increases with the photons' energy, since the probability that the second interaction occurs outside the pixel where the first interaction took place increases as well. ...
Polarimetry is an area of high energy astrophysics which is still relatively unexplored, even though it is recognized that this type of measurement could drastically increase our knowledge of the physics and geometry of high energy sources. For this reason, in the context of the design of a Gamma-Ray Imager based on new hard-X and soft gamma ray focusing optics for the next ESA Cosmic Vision call for proposals (Cosmic Vision 2015-2025), it is important that this capability should be implemented in the principal on-board instrumentation. For the particular case of wide band-pass Laue optics we propose a focal plane based on a thick pixelated CdTe detector operating with high efficiency between 60-600 keV. The high segmentation of this type of detector (1-2 mm pixel size) and the good energy resolution (a few keV FWHM at 500 keV) will allow high sensitivity polarisation measurements (a few % for a 10 mCrab source in 106s) to be performed. We have evaluated the modulation Q factors and minimum detectable polarisation through the use of Monte Carlo simulations (based on the GEANT 4 toolkit) for on and off-axis sources with power law emission spectra using the point spread function of a Laue lens in a feasible configuration.
... Portugal, on the behalf of the GRBM (Gamma Ray Burst Monitor) on board the LOBSTER experiment. The collaboration between these two groups has already gave rise to both extensive Monte Carlo studies [4] ...
Polarimetry in astrophysics had no much development in the hard X- and soft γ-ray energy range; in fact no dedicated polarimeters had ever been launched in space. Previous Monte Carlo simulations and prototype experimental measurements were made in order to evaluate the polarimetric performances of pixelised CZT matrices. The results showed that CZT based polarimeters are able to perform polarimetric measurements of short duration polarised emissions like gamma ray bursts. On the behalf of a Gamma Ray Burst Monitor (GRBM) proposal for the LOBSTER experiment, approved by ESA for a Phase A study for a future flight (2009) aboard the International Space Station (ISS), we propose a Monte Carlo polarimetric study of the 4 detection units that compose this instrument. Each of these units is a 24×12 matrix of CZT elementary crystals. Each pixel has a cross section of 8×8 mm<sup>2</sup> , therefore a detection unit has an active area of about 184 cm<sup>2</sup>. The detector thickness is 3 mm as baseline, and 5 mm in the case this thickness is crucial to exploit the GRBM as polarimeter as well. The 4 detection units have a rectangular FOV of 55°×35° FVVHM and their axes misaligned with each other by 45° in the direction perpendicular to the ISS motion and 10° along the ISS direction of motion. Energy dependent polarimetric Q factor and detection efficiencies will be determined. Expected GRBM minimum detectable polarisation will be presented and discussed.
... Within the scope of building a dedicated polarimeter to study hard X-and soft gamma-ray celestial emissions, a collaboration project has been developed between the IASF -Sezione di Bologna (Istituto di Astrofisica Spaziale e Fisica Cosmica), CNR, Italy and the Departamento de Física da Universidade de Coimbra, Portugal. An instrument was proposed for hard X-ray and soft gamma ray polarimetry in astrophysics, known as the CIPHER (Coded Imager and Polarimeter for High Energy Radiation) telescope [4]. The novel design of this telescope, using a 32×32 matrix of thick CdTe micro-spectrometers as the detection plane will allow this type of measurement to be performed for the first time by a dedicated instrument. ...
High energy polarimetry in astrophysics is a quite unexplored field, as well as optical solutions for polarimeters. Herein we propose and compare two different CdTe based polarimeter optics for X-ray and γ-ray astrophysics. The first configuration that we propose is a CdTe pixelised detector for multilayer telescope operating between 10 and 100 keV. In this case we are dealing with a 2 to 5 mm thick and small area detector with a very fine spatial resolution: pixel pitch of 0.5 mm, because typically the point spread function is of the order of 2-3 mm (FWHM). The disadvantages of this solution are mainly two: the effect of the multilayer mirror reflection on the incoming photons polarisation state and the limited energy range for Compton scattering in CdTe. The second configuration proposed is a CdTe thick pixelised matrix detector for Laue optics based telescope. In this case, it is required better detection efficient at high energy (from 60 to 400 keV) therefore a CdTe thickness from 3 to 10 mm is required, but for pixel scales of the order of a few mm because the expected point spread function is about 30 mm (FWHM) for photon energies of about 200 keV. Modulation Q factors and minimum detectable polarisation are calculated from Monte Carlo simulation of the two type CdTe focal plane detector for the Crab pulsar, as on axes reference source using the analytical response of the two optics type.
The flexible interchange transport system (FITS) file format has become the de facto standard for sharing, analyzing, and archiving astronomical data over the last four decades. FITS was adopted by astronomers in the early 1980s to overcome incompatibilities between operating systems. On the back of FITS’ success, astronomical data became both backward compatible and easily shareable. However, new advances in the astronomical instrumentation, computational technologies, and analytic techniques have resulted in new data that do not work well within the traditional FITS format. Tensions have arisen between the desire to update the format to meet new analytic challenges and adherence to the original edict for the FITS file format to be backward compatible. We examine three inflection points in the governance of FITS: first, initial development and success, second, widespread acceptance and governance by the working group, and third, the challenges to FITS in a new era of increasing data and computational complexity within astronomy.
