An Overview of X-Ray Polarimetry of Astronomical Sources
06 Mar 2018-Vol. 6, Iss: 1, pp 33
TL;DR: The Imaging X-ray Polarimetry Explorer (IXPE) as mentioned in this paper was selected as a NASA Astrophysics Small Explorers Mission and is currently scheduled to launch in April of 2021.
Abstract: We review the history of astronomical X-ray polarimetry based on the author’s perspective, beginning with early sounding-rocket experiments by Robert Novick at Columbia University and his team, of which the author was a member. After describing various early techniques for measuring X-ray polarization, we discuss the polarimeter aboard the Orbiting Solar Observatory 8 (OSO-8) and its scientific results. Next, we describe the X-ray polarimeter to have flown aboard the ill-fated original Spectrum-X mission, which provided important lessons on polarimeter design, systematic effects, and the programmatics of a shared focal plane. We conclude with a description of the Imaging X-ray Polarimetry Explorer (IXPE) and its prospective scientific return. IXPE, a partnership between NASA and ASI, has been selected as a NASA Astrophysics Small Explorers Mission and is currently scheduled to launch in April of 2021.
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Washington University in St. Louis1, Ehime University2, Rice University3, Osaka University4, Hiroshima University5, Royal Institute of Technology6, University of New Hampshire7, Okinawa Institute of Science and Technology8, Goddard Space Flight Center9, Wallops Flight Facility10, Tokyo Metropolitan University11, United States Naval Research Laboratory12
TL;DR: The XL-Calibur mission as mentioned in this paper is a hard X-ray (15-80 keV) polarimetry mission operating from a stabilised balloon-borne platform in the stratosphere.
32 citations
TL;DR: In this paper, the authors revisited the problem of assessing the spectro-polarimetric properties of magnetar persistent emission and found that X-ray polarimetry will allow to detect QED vacuum effects for all the emission models and to discriminate among them.
Abstract: Magnetars are believed to host the strongest magnetic fields in the present universe ($B\gtrsim10^{14}$ G) and the study of their persistent emission in the X-ray band offers an unprecendented opportunity to gain insight into physical processes in the presence of ultra-strong magnetic fields. Up to now, most of our knowledge about magnetar sources came from spectral analysis, which allowed to test the resonant Compton scattering scenario and to probe the structure of the star magnetosphere. On the other hand, radiation emitted from magnetar surface is expected to be strongly polarized and its observed polarization pattern bears the imprint of both scatterings onto magnetospheric charges and QED effects as it propagates in the magnetized vacuum around the star. X-ray polarimeters scheduled to fly in the next years will finally allow to exploit the wealth of information stored in the polarization observables. Here we revisit the problem of assessing the spectro-polarimetric properties of magnetar persistent emission. At variance with previous investigations, proper account for more physical surface emission models is made by considering either a condensed surface or a magnetized atmosphere. Results are used to simulate polarimetric observations with the forthcoming Imaging X-ray Polarimetry Explorer (IXPE). We find that X-ray polarimetry will allow to detect QED vacuum effects for all the emission models we considered and to discriminate among them.
23 citations
Cites methods from "An Overview of X-Ray Polarimetry of..."
...…time of OSO8 (Weisskopf et al. 1978), polarimetric measures at X-rays relied only on rocket and balloon experiments (see Bellazzini et al. 2010; Weisskopf 2018, for an overview), with the exception of the small GPD detector PolarLight (Feng et al. 2019), which flew in 2018 on board of the…...
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19 citations
11 Jan 2021-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: In this article, a weighted maximum likelihood combination of predictions from a deep ensemble of ResNet convolutional neural networks, trained on Monte Carlo event simulations, is used for enhancing the sensitivity of X-ray telescopic observations with imaging polarimeters.
Abstract: We present a method for enhancing the sensitivity of X-ray telescopic observations with imaging polarimeters, with a focus on the gas pixel detectors (GPDs) to be flown on the Imaging X-ray Polarimetry Explorer (IXPE). Our analysis determines photoelectron directions, X-ray absorption points and X-ray energies for 1-9 keV event tracks, with estimates for both the statistical and model (reconstruction) uncertainties. We use a weighted maximum likelihood combination of predictions from a deep ensemble of ResNet convolutional neural networks, trained on Monte Carlo event simulations. We define a figure of merit to compare the polarization bias–variance trade-off in track reconstruction algorithms. For power-law source spectra, our method improves on the current planned IXPE analysis (and previous deep learning approaches), providing ∼ 45 % increase in effective exposure times. For individual energies, our method produces 20%–30% absolute improvements in modulation factor for simulated 100% polarized events, while keeping residual systematic modulation within 1 σ of the finite sample minimum. Absorption point location and photon energy estimates are also significantly improved. We have validated our method with sample data from real GPD detectors.
14 citations
TL;DR: A review of the recent polarimetry findings and science potential of hard X-ray polarimetric along with possible improvements in the measurement techniques can be found in this article, where the authors focus on the recent Polarimetry observations and their science potential.
Abstract: The last decade has seen a leapfrog in the interest of X-ray polarimetry with a number of new polarization measurements in hard X-rays from AstroSat, POLAR, GAP, and PoGO+. The measurements provide some interesting insights into various astrophysical phenomena such as coronal geometry and disk–jet connection in black hole X-ray binaries, hard X-ray emission mechanism in pulsars, and gamma-ray bursts. They also highlight an increase in polarization with energy, which makes hard X-ray polarimetry extremely appealing. There are a number of confirmed hard X-ray polarimetry experiments which along with the existing instruments (AstroSat and INTEGRAL) make this field further exciting. Polarization experiments may also see significant progress in sensitivity with new developments in scintillator readouts, active pixel sensors, and cadmium zinc telluride detectors. In particular, the advent of hard X-ray focusing optics will enable the designing of compact focal plane polarimeters with a multifold enhancement in sensitivity. In this review, we will focus on the recent polarimetry findings and science potential of hard X-ray polarimetry along with possible improvements in the measurement techniques.
12 citations
References
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TL;DR: In this paper, Chandra X-ray images with the Advanced CCD Imaging Spectrometer spectroscopy array (ACIS-S) showed a striking richness of Xray structure at a resolution comparable to that of the best ground-based visible-light observations.
Abstract: The Chandra X-Ray Observatory observed the Crab Nebula and pulsar during orbital calibration. Zeroth-order images with the High-Energy Transmission Grating (HETG) readout by the Advanced CCD Imaging Spectrometer spectroscopy array (ACIS-S) show a striking richness of X-ray structure at a resolution comparable to that of the best ground-based visible-light observations. The HETG-ACIS-S images reveal, for the first time, an X-ray inner ring within the X-ray torus, the suggestion of a hollow-tube structure for the torus, and X-ray knots along the inner ring and (perhaps) along the inward extension of the X-ray jet. Although complicated by instrumental effects and the brightness of the Crab Nebula, the spectrometric analysis shows systematic variations of the X-ray spectrum throughout the nebula.
384 citations
TL;DR: The development of an instrument that makes X-ray polarimetry possible is reported, and the factor of 100 improvement in sensitivity that is achieved will allow direct exploration of the most dramatic objects of theX-ray sky.
Abstract: The study of astronomical objects using electromagnetic radiation involves four basic observational approaches: imaging, spectroscopy, photometry (accurate counting of the photons received) and polarimetry (measurement of the polarizations of the observed photons). In contrast to observations at other wavelengths, a lack of sensitivity has prevented X-ray astronomy from making use of polarimetry. Yet such a technique could provide a direct picture of the state of matter in extreme magnetic and gravitational fields1,2,3,4,5,6, and has the potential to resolve the internal structures of compact sources that would otherwise remain inaccessible, even to X-ray interferometry7. In binary pulsars, for example, we could directly ‘see’ the rotation of the magnetic field and determine if the emission is in the form of a ‘fan’ or a ‘pencil’ beam1,8. Also, observation of the characteristic twisting of the polarization angle in other compact sources would reveal the presence of a black hole9,10,11,12. Here we report the development of an instrument that makes X-ray polarimetry possible. The factor of 100 improvement in sensitivity that we have achieved will allow direct exploration of the most dramatic objects of the X-ray sky.
356 citations
TL;DR: The linear X-ray polarization of the Crab Nebula has been precisely measured at 2.6 keV and 5.2 keV with the OSO 8 graphite crystal polarimeters as discussed by the authors.
Abstract: The linear X-ray polarization of the Crab Nebula has been precisely measured at 2.6 keV and 5.2 keV with the OSO 8 graphite crystal polarimeters. The 1.4 ms time resolution of these instruments permitted the removal of any contribution to the polarization from the pulsar. The nebular polarization is 19.2% plus or minus 1.0% at a position angle of 156.4 plus or minus 1.4 deg at 2.6 keV. At 5.2 keV the corresponding results are 19.5% plus or minus 2.8% at 152.6 plus or minus 4.0 deg.
323 citations
"An Overview of X-Ray Polarimetry of..." refers background in this paper
...4◦ [4] measured east of north, satisfyingly in agreement with the previous measurement and leaving no doubt as to the synchrotron origin of the X-rays....
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...4° [4] measured east of north, satisfyingly in agreement with the previous measurement and leaving no doubt as to the synchrotron origin of the X-rays....
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TL;DR: Zeroth-order images with the High-Energy Transmission Grating (HETG) readout by the Advanced CCD Imaging Spectrometer spectroscopy array (ACIS-S) show a striking richness of X-ray structure at a resolution comparable to that of the best ground-based visible-light observations.
Abstract: The Chandra X-ray Observatory observed the Crab Nebula and Pulsar during orbital calibration. Zeroth-order images with the High-Energy Transmission Grating (HETG) read-out by the Advanced CCD Imaging Spectrometer spectroscopy array (ACIS-S) show a striking richness of x-ray structure, at a resolution comparable to that of the best ground-based visible-light observations. The HETG--ACIS-S images reveal, for the first time, an x-ray inner ring within the x-ray torus, the suggestion of a hollow-tube structure for the torus, and x-ray knots along the inner ring and (perhaps) along the inward extension of the x-ray jet. Although complicated by instrumental effects and the brightness of the Crab Nebula, the spectrometric analysis shows systematic variations of the x-ray spectrum throughout the Nebula.
304 citations
"An Overview of X-Ray Polarimetry of..." refers result in this paper
...The results are shown in Figure 6 and the mean optical position angle is consistent with the OSO-8 X-ray measurements when the nebular siz is in the range of 32–3 arcseconds, in excell nt agreement with the size observed with the Chandra X-ray Observatory [6]....
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...The results are shown in Figure 6 and the mean optical position angle is consistent with the OSO-8 X-ray measurements when the nebular size is in the range of 32–38 arcseconds, in excellent agreement with the size observed with the Chandra X-ray Observatory [6]....
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TL;DR: In this article, a review of the gamma-ray emission mechanisms is presented with the emphasis on their detectable characteristics and the potential astronomical sites in which these emission mechanisms may be at work are discussed.
Abstract: The analysis of compact astronomical objects has generally dealt with the physical properties of the source within a two-parameter space, which is defined by the spectral characteristics and time variability. This approach often leads to the situation whereby two or more very different models can explain the observations successfully. Polarimetric observations have the diagnostic potential to discriminate between the different compact source models and can offer a unique insight into the geometrical nature of the emission zones. To date, however, no polarization observation in the gamma-ray energy domain has been successfully performed, due to the difficulties in making polarimetric measurements in this high-energy region of the spectrum. In this paper the polarized gamma-ray emission mechanisms are reviewed with the emphasis on their detectable characteristics. Potential astronomical sites in which these emission mechanisms may be at work are discussed. Observational results obtained in other wavebands and theoretical predications made for some of the most likely astronomical sources of polarization are reviewed. Compton polarimetry has long been used in the field of nuclear gamma-ray spectroscopy in the laboratory. The operational principle behind all generations of nuclear gamma-ray polarimeters has been to measure the asymmetry in the azimuthal distribution of the scattered photons. However none of the polarimeters designed for laboratory experiments will be sensitive enough to observe even the strongest astronomical source. In the past few years there have been a number of innovative developments aimed at the construction of astronomical gamma-ray polarimeters, either as dedicated experiments or in missions with polarimetric capability. The designs of all the polarimeters are based on either discrete or continuous position sensitive detector planes. In this paper the data analysis techniques associated with this type of polarimeter are discussed as well as methods of removing some of the systematic effects introduced by a non-ideal detector response function and observation conditions. Laboratory tests of these new polarimetric techniques are reviewed. They demonstrate the feasibility of building a suitably sensitive astronomical gamma-ray polarimeter. Optimization of the design of pixellated detector array based polarimeters is also addressed. The INTEGRAL mission, which is to be launched by ESA in the year 2001, is the most likely telescope to perform the first successful gamma-ray polarization observation. The polarimetric characteristics of the two main instruments on board INTEGRAL are evaluated and their sensitivities to a wide range of potentially polarized gamma-ray sources are estimated.
276 citations
"An Overview of X-Ray Polarimetry of..." refers background in this paper
...emporal coincidence. As per the Klein–Nishina formula, such scattering angles will be modulated by the polarization of the incident photon beam, following a sinusoidal distribution with a 180 period [6]. The scintillators, type EJ-204 from Eljen Technology, are 12 cm tall and about 3 cm across, with a hexagonal cross section, allowing them to be tightly packed in a honeycomb structure. Copper tubes,...
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