Circular Polarization Measurements Research Articles

Circular Polarization of Simulated Images of Black Holes

Models of the resolved Event Horizon Telescope (EHT) sources Sgr A* and M87* are constrained by observations at multiple wavelengths, resolutions, polarizations, and time cadences. In this paper, we compare unresolved circular polarization (CP) measurements to a library of models, where each model is characterized by a distribution of CP over time. In the library, we vary the spin of the black hole, the magnetic field strength at the horizon (i.e., both SANE and magnetically arrested disk or MAD models), the observer inclination, a parameter for the maximum ion–electron temperature ratio assuming a thermal plasma, and the direction of the magnetic field dipole moment. We find that Atacama Large Millimeter/submillimeter Array (ALMA) observations of Sgr A* are inconsistent with all edge-on (i = 90°) models. Restricting attention to the MAD models favored by earlier EHT studies of Sgr A*, we find that only models with magnetic dipole moment pointing away from the observer are consistent with ALMA data. We also note that in 26 of the 27 passing MAD models, the accretion flow rotates clockwise on the sky. We provide a table of the means and standard deviations of the CP distributions for all model parameters, along with their trends.

Circular polarization measurement for individual gamma rays in capture reactions with intense pulsed neutrons

Circular polarization measurement for individual gamma rays in capture reactions with intense pulsed neutrons

Characterization of a half-wave plate for cosmic microwave background circular polarization measurement with POLARBEAR.

A half-wave plate (HWP) is often used as a modulator to suppress systematic error in the measurements of cosmic microwave background (CMB) polarization. A HWP can also be used to measure circular polarization (CP) through its optical leakage from CP to linear polarization. The CP of the CMB is predicted from various sources, such as interactions in the Universe and extension of the standard model. Interaction with supernova remnants of population III stars is one of the brightest CP sources. Thus, the observation of the CP of CMB is a new tool for searching for population III stars. In this paper, we demonstrate the improved measurement of the leakage coefficient using the transmission measurement of an actual HWP in the laboratory. We measured the transmittance of linearly polarized light through the HWP used in Polarbear in the frequency range of 120-160GHz. We evaluate the properties of the HWP by fitting the data with a physical model using the Markov Chain Monte Carlo method. We then estimate the band-averaged CP leakage coefficient using the physical model. We find that the leakage coefficient strongly depends on the spectra of CP sources. We thus calculate the maximum fractional leakage coefficient from CP to linear polarization as 0.133 ± 0.009 in the Rayleigh-Jeans spectrum. The nonzero value shows that Polarbear has a sensitivity to CP. Additionally, because we use the bandpass of detectors installed in the telescope to calculate the band-averaged values, we also consider systematic effects in the experiment.

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.

Optical circular polarization induced by axionlike particles in blazars

We propose that the interaction between the axionlike particles (ALPs) and photons can be a possible origin of optical circular polarization (CP) in blazars. Given that there is no definite detection of optical CP at $\sim0.1\%$ level, a rough limit on ALP-photon coupling can be obtained, specifically $g_{a\gamma}\cdot B_\mathrm{T0}\lesssim7.9\times10^{-12}~\mathrm{G\cdot GeV}^{-1}$ for $m_{a}\lesssim 10^{-13}~\mathrm{eV}$, depending on the magnetic field configuration of the blazar jet. Obviously, for the blazar models with a larger magnetic field strength, such as hadronic radiation models, this constraint could be more stringent. We also perform a dedicated analysis of the tentative observations of optical CP in two blazars, namely 3C 66A and OJ 287, and we find that these observations could be explained by the ALP-photon mixing with $g_{a\gamma} \sim 10^{-11}~\mathrm{GeV}^{-1}$. As an outlook, our analysis can be improved by further research on the radiation models of blazars and high-precision joint measurements of optical CP and linear polarization.

Linear and Circular Polarimetry of the Optically Bright Relativistic Tidal Disruption Event AT 2022cmc

Tidal disruption events (TDEs) occur when a star orbiting a massive black hole is sufficiently close to being tidally ripped apart by the black hole. AT 2022cmc is the first relativistic TDE that was observed (and discovered) as an optically bright and fast transient, showing signatures of nonthermal radiation induced by a jet that is oriented toward the Earth. In this work, we present optical linear and circular polarization measurements, observed with the Very Large Telescope/FORS2 in the R band (which corresponds to the blue/UV part of the spectrum in the rest frame), ∼7.2 and ∼12.2 rest-frame days after the first detection, respectively, when the light curve of the transient had settled in a bright blue plateau. Both linear and circular polarizations are consistent with zero, p lin = 0.14% ± 0.73%, and p cir = −0.30% ± 0.53%. This is the highest signal-to-noise ratio linear polarization measurement obtained for a relativistic TDE and the first circular polarimetry for such a transient. The nondetection of the linear and circular polarizations is consistent with the scenario of AT 2022cmc being a TDE where the thermal component (disk+outflows) is viewed pole-on, assuming an axially symmetric geometry. The presence and effect of a jet and/or external shocks are, however, difficult to disentangle.

Confocal Raman microscope with versatile dual polarization snapshot acquisition.

In this paper we propose a new strategy towards simultaneous co- and cross-polarized measurements of Raman spectra in a confocal microscope. One of the advantages of this strategy is that it is immune to polarization-dependent efficiency of diffraction gratings. It is shown via linear angle-resolved and circular polarization measurements that the accuracy of these snapshot polarization measurements on solid and liquid samples are in good agreement with available models and data. The interest of simultaneous acquisition of the total Raman response and the degree of polarization is discussed as well.

Polarimetric signatures of hot spots in black hole accretion flows

Context. The flaring events observed in the Sagittarius A* supermassive black hole system can be attributed to the nonhomogeneous nature of the near-horizon accretion flow. Bright regions in this flow may be associated with density or temperature anisotropies, corresponding to so-called “bright spots” or “hot spots.” Such orbiting features may explain observations at infrared wavelengths, as well as recent findings at millimeter wavelengths. Aims. In this work, we study the emission from an orbiting equatorial bright spot, imposed on a radiatively inefficient accretion flow background, to find polarimetric features indicative of the underlying magnetic field structure and other system variables, including inclination angle, spot size, black hole spin, and more. Specifically, we investigate the impact of these parameters on the Stokes 𝒬 − 𝒰 signatures that commonly exhibit a typical double loop (pretzel-like) structure. Methods. Our semi-analytical model, describing the underlying plasma conditions and the orbiting spot, is built within the framework of the numerical radiative transfer code ipole, which calculates synchroton emission at 230 GHz. Results. We showcase the wide variety of 𝒬 − 𝒰 loop signatures and the relation between inner and outer loops. For the vertical magnetic field topology, the inner 𝒬 − 𝒰 loop is explained by the suppression of the synchrotron emission as seen by the distant observer. For the radial and toroidal magnetic field topologies, the inner 𝒬 − 𝒰 loop corresponds to the part of the orbit where the spot is receding with respect to the observer. Conclusions. Based on our models, we conclude that it is possible to constrain the underlying magnetic field topology with an analysis of the 𝒬 − 𝒰 loop geometry, particularly in combination with circular polarization measurements.

Detecting chiral asymmetry in the interstellar medium using propylene oxide

Context.Life is distinctly homochiral. The origins of this homochirality are under active debate. Recently, propylene oxide has been detected in the gas-phase interstellar medium (ISM). The enantiomeric composition of ISM propylene oxide may be probed through circular polarization measurements, but accurate estimates of the circular dichroism properties of the microwave transitions of propylene oxide are not available.Aims.Our aim is to develop a model of the circular dichroic activity in torsion–rotation transitions of closed-shell chiral molecules such as propylene oxide. With this model we can estimate the viability, and optimize the observation strategies, of enantiomeric excess detection in ISM propylene oxide.Methods.Circular dichroism in spectral lines manifests through the simultaneous interaction of an electromagnetic radiation field with the molecular electric dipole moment and magnetic dipole moment. We developed techniques to quantify electric dipole and magnetic dipole moments of torsion–rotation transitions by expanding on earlier modeling of the electric and magnetic dipole properties of single torsion–rotation levels. To model the circular dichroism properties of propylene oxide, we used these techniques in combination with ab initio quantum chemical calculations.Results.The expressions for the dichroic activity of the microwave transitions of torsionally active molecules are derived. We find that the torsional motion of molecules exhibiting internal rotation contributes significantly to the total magnetic moment. We present estimates for the dichroic activity of the torsion–rotation transitions of propylene oxide. We predict that the circular polarization fractions of emission lines of enantiopure propylene oxide relevant to astronomical detections are on the order of 10−6.Conclusions.Due to the low predicted circular polarization fractions, we conclude that enantiomeric characterization of propylene oxide in the gas phase of the ISM is impossible with the current astronomical observation techniques. We suggest that only chiral radical species may be viably employed for purposes of enantiomeric excess detection. We estimate that laboratory experiments may be successful in detecting the enantiomeric composition of a mixture of propylene oxide through microwave dichroism spectroscopy.

On Far-infrared and Submillimeter Circular Polarization

Abstract Interstellar dust grains are often aligned. If the grain alignment direction varies along the line of sight, the thermal emission becomes circularly polarized. In the diffuse interstellar medium, the circular polarization at far-infrared and submillimeter wavelengths is predicted to be very small, and probably unmeasurable. However, circular polarization may reach detectable levels in photodissociation regions viewed through molecular clouds, in infrared dark clouds, and in protoplanetary disks. Measurement of circular polarization could help constrain the structure of the magnetic field in infrared dark clouds, and may shed light on the mechanisms responsible for grain alignment in protoplanetary disks.

Highly sensitive search for magnetic fields in white dwarfs using broad-band circular polarimetry

Circular polarisation measurements of white dwarfs of various ages and spectral types are useful to understand the origin and evolution of the magnetic field in degenerate stars. In the latest stages of white dwarf evolution, when stars are so cool that spectral lines are no longer formed in the normal H- or He-dominated atmospheres, magnetic fields can be probed only by means of circular polarimetry of the continuum. The study of the fields of featureless DC white dwarfs may reveal whether Ohmic decay acts on magnetic white dwarfs, or if magnetic fields continue to be generated even several billion years after white dwarf formation. Compared to spectropolarimetry, broad-band circular polarisation measurements have the advantage of reaching a higher accuracy in the continuum, with the potential of detecting magnetic fields as weak as a fraction of a MG in DC stars, if the telescope size is adequate for the star’s magnitude. Here we present the results of a first (short) observing campaign with the DIPol-UF polarimeter, which we have used to measure broad-band circular polarisation of white dwarfs. Our observing run was in part aimed to fully characterise the instrument, and in part to study the relationship between magnetic field strength (when known from spectropolarimetry) and circular polarisation of the continuum. We also observed a small number of previously unexplored DC white dwarfs, and we present the discovery of two new magnetic white dwarfs of spectral class DC, probably the first discovery of this kind made with broad-band circular polarimetric techniques since the late 1970s. We also discuss the characteristics of our instrument, and predict the level of polarimetric accuracy that may be reached as a function of stellar magnitude, exposure time, and telescope size.

Biosignatures of the Earth

Context.Homochirality is a generic and unique property of life on Earth and is considered a universal and agnostic biosignature. Homochirality induces fractional circular polarization in the incident light that it reflects. Because this circularly polarized light can be sensed remotely, it can be one of the most compelling candidate biosignatures in life detection missions. While there are also other sources of circular polarization, these result in spectrally flat signals with lower magnitude. Additionally, circular polarization can be a valuable tool in Earth remote sensing because the circular polarization signal directly relates to vegetation physiology.Aims.While high-quality circular polarization measurements can be obtained in the laboratory and under semi-static conditions in the field, there has been a significant gap to more realistic remote sensing conditions.Methods.In this study, we present sensitive circular spectropolarimetric measurements of various landscape elements taken from a fast-moving helicopter.Results.We demonstrate that during flight, within mere seconds of measurements, we can differentiate (S∕N> 5) between grass fields, forests, and abiotic urban areas. Importantly, we show that with only nonzero circular polarization as a discriminant, photosynthetic organisms can even be measured in lakes.Conclusions.Circular spectropolarimetry can be a powerful technique to detect life beyond Earth, and we emphasize the potential of utilizing circular spectropolarimetry as a remote sensing tool to characterize and monitor in detail the vegetation physiology and terrain features of Earth itself.

New Constraints on Lorentz Invariance Violation from Combined Linear and Circular Optical Polarimetry of Extragalactic Sources

Expanding on our prior efforts to search for Lorentz invariance violation (LIV) using the linear optical polarimetry of extragalactic objects, we propose a new method that combines linear and circular polarization measurements. While existing work has focused on the tendency of LIV to reduce the linear polarization degree, this new method additionally takes into account the coupling between photon helicities induced by some models. This coupling can generate circular polarization as light propagates, even if there is no circular polarization at the source. Combining significant detections of linear polarization of light from extragalactic objects with the absence of the detection of circular polarization in most measurements results in significantly tighter constraints regarding LIV. The analysis was carried out in the framework of the Standard-Model Extension (SME), an effective field theory framework to describe the low-energy effects of an underlying fundamental quantum gravity theory. We evaluate the performance of our method by deriving constraints on the mass dimension d=4 CPT-even SME coefficients from a small set of archival circular and linear optical polarimetry constraints and compare them to similar constraints derived in previous works with far larger sample sizes and based on linear polarimetry only. The new method yielded constraints that are an order of magnitude tighter even for our modest sample size of 21 objects. Based on the demonstrated gain in constraining power from scarce circular data, we advocate for the need for future extragalactic circular polarization surveys.

Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: A Measurement of Circular Polarization at 40 GHz

Abstract We report measurements of circular polarization from the first two years of observation with the 40 GHz polarimeter of the Cosmology Large Angular Scale Surveyor (CLASS). CLASS is conducting a multi-frequency survey covering 75% of the sky from the Atacama Desert designed to measure the cosmic microwave background (CMB) linear E and B polarization on angular scales 1° ≲ θ ≤ 90°, corresponding to a multipole range of 2 ≤ ℓ ≲ 200. The modulation technology enabling measurements of linear polarization at the largest angular scales from the ground, the Variable-delay Polarization Modulator, is uniquely designed to provide explicit sensitivity to circular polarization (Stokes V). We present a first detection of circularly polarized atmospheric emission at 40 GHz that is well described by a dipole with an amplitude of when observed at an elevation of 45°, and discuss its potential impact on the recovery of linear polarization by CLASS. Filtering the atmospheric component, CLASS places a 95% confidence upper limit of to on for , representing an improvement by two orders of magnitude over previous CMB limits.

An HF Software‐Defined Radar to Study the Ionosphere

AbstractIn this paper, a novel design and implementation of a software‐defined high‐frequency ionospheric radar, the Penn State Ionospheric Radar Imager (PIRI), is described. Furthermore, preliminary results produced by the system (located at 40.71° N, 77.97° W) are presented. PIRI is designed to be a modest and low‐cost radar system, which is composed mostly of commercial‐off‐the‐shelf products and utilizing open‐source software to perform pulse generation, pulse coding, downconversion, data acquisition, and signal processing. It is designed to be mobile, as it can easily be deployed at temporary locations to study local ionospheric disturbances. For the results presented herein, the radar operating frequency was 5.125 MHz. However, as the system is software defined and short active receive antennas are used, only the transmit antenna needs to be changed to operate over the entire high‐frequency (HF) band. The two orthogonal receive antennas enable both linear and circular polarization measurements. Peak transmit power of the system is 500 W. PIRI is designed to be a modest and cost‐effective alternative to the current standard HF ionospheric sounding systems and can be readily replicated.

Polarization VLBI observations of AGN jets now and into the future

The history of Very Long Baseline Interferometry (VLBI) observations has been characterized predominantly by an ongoing quest for increasingly high resolution and sensitivity. VLBI monitoring of relatively large samples of Active Galactic Nuclei (AGNs) with uniform quality and linear polarization sensitivity are now available at the moderately high frequencies of 15 and 43 GHz. This has enabled considerable advances in our understanding of the relativistic jets of AGNs, but gaps in the available observational material remain, which must be addressed in future VLBI polarization observations. Linear polarization observations at frequencies above 43 GHz remain non-routine, and the availability of multi-frequency and circular polarization measurements is still limited. It is of interest both to push inward toward the jet base and to study details of the jets themselves on parsec scales, such as magnetic field structures along and across the jets, which are intrinsically related to their formation, launching and propagation. Requirements for future VLBI polarization observations are considered, highlighting the key role that can be played by space VLBI observations. Unique opportunities are offered by relatively low-frequency space VLBI observations that are sensitive to effects such as Faraday rotation, opacity, and low-frequency absorption.

Conversion Measure of Faraday Rotation–Conversion with Application to Fast Radio Bursts

Abstract Faraday rotation–conversion is the simultaneous rotation of all three Stokes polarization parameters, Q, U, and V, as an electromagnetic wave propagates through a magnetized plasma. In this regime, the Faraday plasma screen is characterized by more than just a rotation measure. We define the conversion measure that characterizes the wavelength-dependent conversion between the linear and circular polarization. In a cold plasma, the conversion occurs at the localized regions along the wave’s path, where the large-scale magnetic field is perpendicular to the propagation direction. We show that the number of these regions along the line of sight through the screen, and their individual contributions to the conversion measure, can be inferred from the polarization measurements. We argue that the simultaneous measurement of wavelength-dependent linear and circular polarization might give an important insight into the magnetic-field geometry of the Faraday screen in FRB 121102 and other repeating fast radio bursts.

Determining the composition of radio plasma via circular polarization: the prospects of the Cygnus A hot spots

The composition of the relativistic plasma produced in active galactic nuclei and ejected via powerful jets into the interstellar/intergalactic medium is still a major unsettled issue. It might be a positron-electron plasma in case the plasma was created by pair production in the intense photon fields near accreting super-massive black holes. Alternatively, it might be an electron-proton plasma in case magnetic fields lift and accelerate the thermal gas of accretion discs into relativistic jets as the recent detection of γ-rays from blazars indicates. Despite various attempts to unambiguously establish the composition of the relativistic jets, this remains a major unknown. Here, we propose a possible way to settle the question via sensitive measurements of circular polarization (CP) in the radio emission of the hot spots of bright radio galaxies like Cygnus A. The CP of synchrotron emission is determined by the circular motions of the radiating relativistic leptons. In case of charge symmetric energy spectra of an electron-positron plasma, it should be exactly zero. In case of an electron-proton plasma the electrons imprint their gyration onto the CP and we expect the hot spots of Cygnus A to exhibit a fractional CP at a level of 10−4(ν/GHz)−1/2, which is challenging to measure, but not completely unfeasible.

Imaging linear and circular polarization features in leaves with complete Mueller matrix polarimetry

Spectropolarimetry of intact plant leaves allows to probe the molecular architecture of vegetation photosynthesis in a non-invasive and non-destructive way and, as such, can offer a wealth of physiological information. In addition to the molecular signals due to the photosynthetic machinery, the cell structure and its arrangement within a leaf can create and modify polarization signals. Using Mueller matrix polarimetry with rotating retarder modulation, we have visualized spatial variations in polarization in transmission around the chlorophyll a absorbance band from 650 nm to 710 nm. We show linear and circular polarization measurements of maple leaves and cultivated maize leaves and discuss the corresponding Mueller matrices and the Mueller matrix decompositions, which show distinct features in diattenuation, polarizance, retardance and depolarization. Importantly, while normal leaf tissue shows a typical split signal with both a negative and a positive peak in the induced fractional circular polarization and circular dichroism, the signals close to the veins only display a negative band. The results are similar to the negative band as reported earlier for single macrodomains. We discuss the possible role of the chloroplast orientation around the veins as a cause of this phenomenon. Systematic artefacts are ruled out as three independent measurements by different instruments gave similar results. These results provide better insight into circular polarization measurements on whole leaves and options for vegetation remote sensing using circular polarization.

Probing the Magnetic Field Structure in on Black Hole Horizon Scales with Polarized Radiative Transfer Simulations

Abstract Magnetic fields are believed to drive accretion and relativistic jets in black hole accretion systems, but the magnetic field structure that controls these phenomena remains uncertain. We perform general relativistic (GR) polarized radiative transfer of time-dependent three-dimensional GR magnetohydrodynamical simulations to model thermal synchrotron emission from the Galactic Center source Sagittarius A* (Sgr A*). We compare our results to new polarimetry measurements by the Event Horizon Telescope (EHT) and show how polarization in the visibility (Fourier) domain distinguishes and constrains accretion flow models with different magnetic field structures. These include models with small-scale fields in disks driven by the magnetorotational instability as well as models with large-scale ordered fields in magnetically arrested disks. We also consider different electron temperature and jet mass-loading prescriptions that control the brightness of the disk, funnel-wall jet, and Blandford–Znajek-driven funnel jet. Our comparisons between the simulations and observations favor models with ordered magnetic fields near the black hole event horizon in Sgr A*, though both disk- and jet-dominated emission can satisfactorily explain most of the current EHT data. We also discuss how the black hole shadow can be filled-in by jet emission or mimicked by the absence of funnel jet emission. We show that stronger model constraints should be possible with upcoming circular polarization and higher frequency (349 GHz) measurements.