Precise Polarization Measurements Research Articles

High Hard X-Ray Polarization in Cygnus X-1 Confined to the Intermediate Hard State: Evidence for a Variable Jet Component

Cygnus X-1, the well-known accreting black hole system, exhibits several observational features hinting at an intricate interplay between the accretion disk, its atmosphere known as the corona, and the putative relativistic jet. It has been extensively studied using all available observational methods, including using the newly available technique of sensitive X-ray polarimetry. X-ray polarization characteristics are distinct for coronal and jet emissions. The low X-ray polarization measured below ∼100 keV is understood as arising from the corona. In contrast, the high polarization measurements reported above ∼400 keV required a separate jet-dominated spectral component, which spectroscopy does not demonstrate conclusively. Here we report precise polarization measurements in the 100–380 keV region made during three different subclasses of spectral states of the source using the CZTI instrument onboard AstroSat. A high polarization (23% ± 4%) is found mainly in the Intermediate Hard State of the source, and the energy-resolved measurements smoothly connect the coronal and the jet regimes. When high polarization is observed, the simultaneous spectral data hints at a separate power-law component above 100 keV. We examine the possible sources of this energy-dependent high polarization in Cygnus X-1.

Hyperfine-induced effects on the linear polarization of the magnetic-quadrupole lines of spin-1/2 Be-like ions excited by electron impact

The linear polarization of magnetic-quadrupole emission lines 1s22s2p3/23P2→1s22s21S0 and 1s2s22p3/23P2→1s22s21S0 following electron-impact excitation of Be-like ions with nuclear spin I=1/2 is studied by employing the density-matrix theory and the relativistic distorted-wave theory. To explore the effects of the hyperfine interaction, detailed calculations are performed for the polarization of the magnetic-quadrupole lines emitted from AXe50+ (A = 125, 127, and 129) and ATl77+ (A = 187, 205, and 207) ions with different nuclear magnetic dipole moments μI at a series of impact electron energies. It is shown that the hyperfine interaction strongly lowers the polarization of the 1s22s2p3/23P2→1S0 line at all considered impact energies, while its effects behave less and less prominent with increasing impact energy for the 1s2s22p3/23P2→1S0 line. In addition, we also find that the polarization of the 1s22s2p3/23P2→1S0 line is much more sensitive to μI than that of the 1s2s22p3/23P2→1S0 line. In particular, for the 1s22s2p3/23P2→1S0 line of spin-1/2 Xe50+ ions, the differences in the polarization among the isotopes are significant. These findings indicate that precise polarization measurements of magnetic-quadrupole lines of Be-like ions are expected to be used for exploring the hyperfine interaction and even the nuclear magnetic dipole moment of nonzero-spin isotopes.

Proton irradiation of plastic scintillator bars for POLAR-2

POLAR-2, a plastic scintillator based Compton polarimeter, is currently under development and planned for a launch to the China Space Station in 2025. It is intended to shed a new light on our understanding of Gamma-Ray Bursts by performing high precision polarization measurements of their prompt emission. The instrument will be orbiting at an average altitude of 383 km with an inclination of 42° and will be subject to background radiation from cosmic rays and solar events. In this work, we tested the performance of plastic scintillation bars, EJ-200 and EJ-248M from Eljen Technology, under space-like conditions, that were chosen as possible candidates for POLAR-2. Both scintillator types were irradiated with 58 MeV protons at several doses from 1.89 Gy(corresponding to about 13 years in space for POLAR-2) up to 18.7 Gy, that goes far beyond the expected POLAR-2 life time. Their respective properties, expressed in terms of light yield, emission and absorption spectra, and activation analysis due to proton irradiation are discussed. Scintillators activation analyses showed a dominant contribution of β + decay with a typical for this process gamma-ray energy line of 511 keV.

Probing axions via light circular polarization and event horizon telescope

The impact of axion-like particles on the light polarization around the horizon of supermassive black hole (SMBH) is discussed in the light of the latest polarization measurement of the Event Horizon Telescope (EHT). We investigate different sources of the polarization due to axion interaction with photons and the magnetic field of SMBH. These can modify the linear and circular polarization parameters of the emitted light. We have shown that a significant circular polarization can be produced via the photon scattering from the background magnetic field with axions as off-shell particles. This can further constrain the parameter space of ultralight axion-like particles and their couplings with photons. The future precise measurements of circular polarization can probe the features of ultralight axions in the near vicinity of SMBH.

Discordances in Cosmology and the Violation of Slow-Roll Inflationary Dynamics.

We identify examples of single field inflationary trajectories beyond the slow-roll regime that improve the fit to Planck 2018 data compared to a baseline Λ cold dark matter model with power law form of primordial spectrum and at the same time alleviate existing tensions between different datasets in the estimate of cosmological parameters such as H_{0} and S_{8}. A damped oscillation in the first Hubble flow function-or equivalently a feature in the potential-and the corresponding localized oscillations in the primordial power spectrum partially mimic the improvement in the fit of Planck data due to A_{L} or Ω_{K}. Compared to the baseline model, this model can lead simultaneously to a larger value of H_{0} and a smaller value of S_{8}, a trend that can be enhanced when the most recent SH0ES measurement for H_{0} is combined with Planck and BICEP-Keck 2018 data. Large scale structure data and more precise cosmic microwave background polarization measurements will further provide critical tests of this intermediate fast-roll phase.

Performance forecasts for the primordial gravitational wave detection pipelines for AliCPT-1

AliCPT is the first Chinese cosmic microwave background (CMB) experiment which will make the most precise measurements of the CMB polarization in the northern hemisphere. The key science goal for AliCPT is the detection of primordial gravitational waves (PGWs). It is well known that an epoch of cosmic inflation, in the very early universe, can produce PGWs, which leave an imprint on the CMB in form of odd parity B-mode polarization. In this work, we study the performance of the component separation and parameter estimation pipelines in context of constraining the value of the tensor-to-scalar ratio. Based on the simulated data for one observation season, we compare five different pipelines with different working principles. Three pipelines perform component separation at map or spectra level before estimating r from the cleaned spectra, while the other two pipelines performs a global fit for both foreground parameters and r. We also test different methods to account for the effects of time stream filtering systematics. This work shows that our pipelines provide consistent and robust constraints on the tensor-to-scalar ratio anda consistent sensitivity σ(r) ∼ 0.02. This showcases the potential of precise B-mode polarization measurement with AliCPT-1. AliCPT will provide a powerful opportunity to detect PGWs, which is complementary with various ground-based CMB experiments in the southern hemisphere.

Detailed polarization measurements of the prompt emission of five gamma-ray bursts

Gamma-ray bursts are the strongest explosions in the Universe since the Big Bang, believed to be produced either in forming black holes at the end of massive star evolution or merging of compact objects. Spectral and timing properties of gamma-ray bursts suggest that the observed bright gamma-rays are produced in the most relativistic jets in the Universe; however, the physical properties, especially the structure and magnetic topologies in the jets are still not well known, despite several decades of studies. It is widely believed that precise measurements of the polarization properties of gamma-ray bursts should provide crucial information on the highly relativistic jets. As a result there have been many reports of gamma-ray burst polarization measurements with diverse results, see, however many such measurements suffered from substantial uncertainties, mostly systematic. After the first successful measurements by the GAP and COSI instruments, here we report a statistically meaningful sample of precise polarization measurements, obtained with the dedicated gamma-ray burst polarimeter, POLAR onboard China's Tiangong-2 spacelab. Our results suggest that the gamma-ray emission is at most polarized at a level lower than some popular models have predicted; although our results also show intrapulse evolution of the polarization angle. This indicates that the low polarization degrees could be due to an evolving polarization angle during a gamma-ray burst.

Confronting phantom inflation with Planck data

The latest Planck results are in excellent agreement with the theoretical expectations predicted from standard normal inflation based on slow-roll approximation which assumes equation-of-state $\omega \geq-1$. In this work, we study the phantom inflation ($\omega<-1$) as an alternative cosmological model within the slow-climb approximation using two hybrid inflationary fields. We perform Chain Monte Carlo analysis to determine the posterior distribution and best fit values for the cosmological parameters using Planck data and show that current CMB data does not discriminate between normal and phantom inflation. Interestingly, unlike in normal inflation, $\omega$ in phantom induced inflation evolves very slowly away from $-1$ during the inflation. Furthermore, in contrast to the standard normal inflation for which only upper bound on tensor-to-scalar ratio $r$ are possible, we obtain both upper and lower bounds for the two hybrid fields in the phantom scenario. Finally, we discuss prospects of future high precision polarization measurements and show that it may be possible to establish the dominance of one model over the other.

Precision electron beam polarimetry for next generation nuclear physics experiments

Polarized electron beams have played an important role in scattering experiments at moderate to high beam energies. Historically, these experiments have been primarily targeted at studying hadronic structure — from the quark contribution to the spin structure of protons and neutrons, to nucleon elastic form factors, as well as contributions to these elastic form factors from (strange) sea quarks. Other experiments have aimed to place constraints on new physics beyond the Standard Model. For most experiments, knowledge of the magnitude of the electron beam polarization has not been a limiting systematic uncertainty, with only moderately precise beam polarimetry requirements. However, a new generation of experiments will require extremely precise measurements of the beam polarization, significantly better than 1%. This paper will review standard electron beam polarimetry techniques and possible future technologies, with an emphasis on the ever-improving precision that is being driven by the requirements of electron scattering experiments.

Dielectric metasurfaces for quantum weak measurements

Metasurfaces have shown extraordinary abilities to manipulate the phase and polarization of light and thus hold great promise for applications in photonics and optoelectronics. Here, we propose a unique application of the dielectric metasurface for quantum weak measurements. In our scheme, the dielectric metasurface introduces a tiny phase gradient in the procedure of measurements and keeps the measured system almost undisturbed. The dielectric metasurface may improve and simplify already existing schemes in quantum weak measurements and thereby provide potential applications in precise measurements of phase, polarization, and frequency of light.

High-Reflectivity Coatings for a Vacuum Ultraviolet Spectropolarimeter

Precise polarization measurements in the vacuum ultraviolet (VUV) region are expected to be a new tool for inferring the magnetic fields in the upper atmosphere of the Sun. High-reflectivity coatings are key elements to achieving high-throughput optics for precise polarization measurements. We fabricated three types of high-reflectivity coatings for a solar spectropolarimeter in the hydrogen Lyman- $\upalpha $ (Ly $\upalpha $ ; 121.567 nm) region and evaluated their performance. The first high-reflectivity mirror coating offers a reflectivity of more than 80 % in Ly $\upalpha $ optics. The second is a reflective narrow-band filter coating that has a peak reflectivity of 57 % in Ly $\upalpha $ , whereas its reflectivity in the visible light range is lower than 1/10 of the peak reflectivity ( $\sim 5~\%$ on average). This coating can be used to easily realize a visible light rejection system, which is indispensable for a solar telescope, while maintaining high throughput in the Ly $\upalpha $ line. The third is a high-efficiency reflective polarizing coating that almost exclusively reflects an s-polarized beam at its Brewster angle of 68° with a reflectivity of 55 %. This coating achieves both high polarizing power and high throughput. These coatings contributed to the high-throughput solar VUV spectropolarimeter called the Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP), which was launched on 3 September, 2015.

Full field of view polarization effect measurement and error analysis of non-polarized channels of spaceborne directional polarimetric camera

The optical system of spaceborne directional polarimetric camera that employs an ultra wide-angle lens for its multiangle, a filter wheel for its multispectral and also its multipolarization capability, a CCD itself for its imaging has a certain polarization effect, which can affect the radiometric accuracy of the non-polarized channels of the instrument. The transmittance of the oblique light rays that are incident on the optical element surfaces is sensitive to the orientation of the electric field, contributing to the linear polarization effect of optical system. The precise polarization measurement and calibration of the passive imaging polarimetry are in urgent need to eliminate the instrumental polarization effect and to improve its radiometric accuracy for observation scenes. The non-polarized channel radiometric model considering the linear polarization effect is deduced in detail by analyzing the instrumental principle and optical structures. Moreover, the reasonably simplified model is established based on the actual lens characteristics. A calibration method in which completely linearly polarized light with different kinds of polarization angles irradiates sparsely the instrument within full field of view and subsequently fits the response in the least square sense, is proposed and simulated. In addition, the measured relative errors of the intensity of incident light with different kinds of polarization states caused by the calibration deviations of instrumental principal physical parameters are analyzed and calculated, such as the azimuthal angle of single pixels, explicit optic polarization rate and low frequency spatial relative transmittance. The actual instrumental parameter values and their calibration deviation amounts are acquired by carrying out the laboratory calibration experiment for instrument and combining the least square fitting. Furthermore, the maximum radiometric calibration relative error caused by the deviation of the physical parameter called explicit optic polarization rate is calculated to be 0.4%, fulfilling completely the requirement of radiometric relative accuracy 5% and retaining abundant tolerance. The study provides a theoretical basis and an experimental guidance in high accurately measuring radiation, calibrating and processing data for the instrumental non-polarized channels with full field of view.

Polarization measurement analysis

High precision polarization measurements open new opportunities for the study of the magnetic field structure as traced by polarimetric measurements of the interstellar dust emission. Polarization parameters suffer from bias in the presence of measurement noise. It is critical to take into account all the information available in the data in order to accurately derive these parameters. The goal of this paper is to characterize the bias on the polarization angle dispersion function that is used to study the spatial coherence of the polarization angle. We characterize, for the first time, the bias on the conventional estimator of the polarization angle dispersion function (S hereafter) and show that it can be positive or negative depending on the true value. Monte Carlo simulations are performed in order to explore the impact of the noise properties of the polarization data, as well as the impact of the distribution of the true polarization angles on the bias. We show that in the case where the ellipticity of the noise in (Q, U) varies by less than 10 percent, one can use simplified, diagonal approximation of the noise covariance matrix. In other cases, the shape of the noise covariance matrix should be taken into account in the estimation of S. We also study new estimators such as the dichotomic and the polynomial estimators. Though the dichotomic estimator cannot be directly used to estimate S, we show that, on the one hand, it can serve as an indicator of the accuracy of the conventional estimator and, on the other hand, it can be used for deriving the polynomial estimator. We propose a method for determining the upper limit of the bias on the conventional estimator of S. The method is applicable to any linear polarization data set for which the noise covariance matrices are known.

Vacuum ultraviolet spectropolarimeter design for precise polarization measurements.

Precise polarization measurements in the vacuum ultraviolet (VUV) region provide a new means for inferring weak magnetic fields in the upper atmosphere of the Sun and stars. We propose a VUV spectropolarimeter design ideally suited for this purpose. This design is proposed and adopted for the NASA-JAXA chromospheric lyman-alpha spectropolarimeter (CLASP), which will record the linear polarization (Stokes Q and U) of the hydrogen Lyman-α line (121.567nm) profile. The expected degree of polarization is on the order of 0.1%. Our spectropolarimeter has two optically symmetric channels to simultaneously measure orthogonal linear polarization states with a single concave diffraction grating that serves both as the spectral dispersion element and beam splitter. This design has a minimal number of reflective components with a high VUV throughput. Consequently, these design features allow us to minimize the polarization errors caused by possible time variation of the VUV flux during the polarization modulation and by statistical photon noise.

Polarization measurement analysis

With the forthcoming release of high precision polarization measurements, such as from the Planck satellite, the metrology of polarization needs to be improved. In particular, it is important to have full knowledge of the noise properties when estimating polarization fraction and polarization angle, which suffer from well-known biases. While strong simplifying assumptions have usually been made in polarization analysis, we present a method for including the full covariance matrix of the Stokes parameters in estimates of the distributions of the polarization fraction and angle. We thereby quantified the impact of the noise properties on the biases in the observational quantities and derived analytical expressions for the probability density functions of these quantities that take the full complexity of the covariance matrix into account, including the Stokes I intensity components. We performed Monte Carlo simulations to explore the impact of the noise properties on the statistical variance and bias of the polarization fraction and angle. We show that for low variations (< 10%) of the effective ellipticity between the Q and U components around the symmetrical case the covariance matrix may be simplified as is usually done, with a negligible impact on the bias. For S/Ns with intensity lower than 10, the uncertainty on the total intensity is shown to drastically increase the uncertainty of the polarization fraction but not the relative bias of the polarization fraction, while a 10% correlation between the intensity and the polarized components does not significantly affect the bias of the polarization fraction. We compare estimates of the uncertainties that affect polarization measurements, addressing limitations of the estimates of the S/N, and we show how to build conservative confidence intervals for polarization fraction and angle simultaneously. This study, which is the first in a set of papers dedicated to analysing polarization measurements, focuses on the basic polarization fraction and angle measurements. It covers the noise regime where the complexity of the covariance matrix may be largely neglected in order to perform further analysis. A companion paper focuses on the best estimators of the polarization fraction and angle and on their associated uncertainties.

Multi-element readout of structured HPGe-detectors for high-resolution x-ray spectroscopy using CUBE-preamplifiers

Very recently we have shown that CUBE-preamplifiers developed by XGLab s.r.l. can be used for the readout of single elements of thick structured planar HPGe- and Si(Li)-detectors produced by SEMIKON [1]. In this paper we will present the results of a simultaneous multi-element readout of structured detectors using the same preamplifiers for measuring high-energy x-rays (more than 100 keV) with a comparable energy resolution as for the single-element readout. Several high-purity germanium detectors (HPGe-detectors) with different position sensitive structures on one detector contact have been used for the first tests. In addition to that we have modified an existing 16-pixel HPGe-polarimeter from GSI-Darmstadt with the new readout. The detector elements (7 mm × 7 mm each, arranged in a 4 × 4 matrix) are connected to CUBE-preamplifiers used in pulse-reset mode. The technological progress achieved with this detector system resulting in a significant improved energy resolution will contribute a lot to much more precise polarization measurements of x-rays emitted from atom-ion collisions which are part of the physics program of the SPARC collaboration (Stored Particles Atomic Physics Research Collaboration) at GSI and the future FAIR accelerator facility (Facility for Antiproton and Ion Research).

Prospects of the Hard X-Ray Instrument POLAR to Measure Polarization of Solar Flares

In-depth studies of solar flares emissions and energy releases include analyses of polarization data. Polarization gives clear information about mechanisms and processes leading to electron acceleration and photon production. Despite of many past attempts, the key energy range of hard X-rays was only rarely explored and results were inconclusive. To large extend it was due to greater instrumental complications. Currently several novel polarimeters are either to be employed or under constructions for both balloon and satellite based observations. The novel hard X-ray polarimeter POLAR is an instrument developed by a collaboration between Switzerland, China and Poland. It is primarily designed for high accuracy polarization measurements from the prompt photon emissions of the gamma-ray bursts. The satellite orientation and instrument pointing direction make it also capable for precise measurements of polarization in solar flares. The instrument should fly in near future onboard of the Chinese Space Station TG2.

Highly precise and accurate terahertz polarization measurements based on electro-optic sampling with polarization modulation of probe pulses.

We have developed an electro-optic (EO) sampling method with polarization modulation of probe pulses; this method allows us to measure the direction of a terahertz (THz) electric-field vector with a precision of 0.1 mrad in a data acquisition time of 660 ms using a 14.0-kHz repetition rate pulsed light source. Through combination with a THz time-domain spectroscopy technique, a time-dependent two-dimensional THz electric field was obtained. We used a photoelastic modulator for probe-polarization modulation and a (111)-oriented zincblende crystal as the EO crystal. Using the tilted pulse front excitation method with stable regeneratively amplified pulses, we prepared stable and intense THz pulses and performed pulse-by-pulse analog-to-digital conversion of the signals. These techniques significantly reduced statistical errors and enabled sub-mrad THz polarization measurements. We examined the performance of this method by measuring a wire-grid polarizer as a sample. The present method will open a new frontier of high-precision THz polarization sensitive measurements.

Precise polarization measurements via detection of compton scattered electrons

The Qweak experiment at Jefferson Lab aims to make a 4% measurement of the parity-violating asymmetry in elastic scattering at very low Q2 of a longitudinally polarized electron beam off a proton target. One of the dominant experimental systematic uncertainties in Qweak will result from determining the beam polarization. A new Compton polarimeter was installed in the fall of 2010 to provide a non-invasive and continuous monitoring of the electron beam polarization in Hall C at Jefferson Lab. The Compton-scattered electrons are detected in four planes of diamond micro-strip detectors. We have achieved the design goals of <1% statistical uncertainty per hour and expect to achieve <1% systematic uncertainty.

Novel approaches in the SR beamline design

High brightness third generation x-ray sources bring new experimental possibilities and impose new challenges. Coherent scattering and diffraction-limited microscopy require wave-preserving optics, high-resolution inelastic scattering novel optical elements, where x-ray interferometry or the requirements of precise polarization measurements change the optical layout. With NSLS-II development as an illustration we discuss recent trends in beamline design.