X-ray Polarization Measurements Research Articles

Probing Magnetic Fields in Young Supernova Remnants with IXPE

Synchrotron emission from the shocked regions in supernova remnants provides, through its polarization, crucial details about the magnetic field strength and orientation in these regions. This, in turn, provides information on particle acceleration in these shocks. Due to the rapid losses of the highest-energy relativistic electrons, X-ray polarization measurements allow for investigations of the magnetic field to be carried outvery close to the sites of particle acceleration. Measurements of both the geometry of the field and the levels of turbulence implied by the observed polarization degree thus provide unique insights into the conditions leading to efficient particle acceleration in fast shocks. The Imaging X-ray Polarimetry Explorer (IXPE) has carried out observations of multiple young SNRs, including Cas A, Tycho, SN 1006, and RX J1713.7−3946. In each, significant X-ray polarization detections provide measurements of magnetic field properties that show some common behavior but also considerable differences between these SNRs. Here, we provide a summary of results from IXPE studies of young SNRs, providing comparisons between the observed polarization and the physical properties of the remnants and their environments.

X-ray polarization measurement of the gold standard of radio-quiet active galactic nuclei: NGC 1068

Context. NGC 1068 is the most observed radio-quiet active galactic nucleus (AGN) in polarimetry, yet its high-energy polarization has never been probed before due to a lack of dedicated polarimeters. Aims. Using the first X-ray polarimeter sensitive enough to measure the polarization of AGNs, we want to probe the orientation and geometric arrangement of (sub)parsec-scale matter around the X-ray source. Methods. We used the Imaging X-ray Polarimetry Explorer (IXPE) satellite to measure, for the first time, the 2–8 keV polarization of NGC 1068. We pointed IXPE at the target for a net exposure time of 1.15 Ms, in addition to using two Chandra snapshots of ∼10 ks each in order to account for the potential impact of several ultraluminous X-ray sources (ULXs) within IXPE’s field of view. Results. We measured a 2–8 keV polarization degree of 12.4% ± 3.6% and an electric vector polarization angle of 101° ± 8° at a 68% confidence level. If we exclude the spectral region containing bright Fe K lines and other soft X-ray lines where depolarization occurs, the polarization fraction rises to 21.3% ± 6.7% in the 3.5–6.0 keV band, with a similar polarization angle. The observed polarization angle is found to be perpendicular to the parsec-scale radio jet. Using a combined Chandra and IXPE analysis plus multiwavelength constraints, we estimated that the circumnuclear “torus” may sustain a half-opening angle of 50–55° (from the vertical axis of the system). Conclusions. Thanks to IXPE, we have measured the X-ray polarization of NGC 1068 and found comparable results, both in terms of the polarization angle orientation with respect to the radio jet and the torus half-opening angle, to the X-ray polarimetric measurement achieved for the other archetypal Compton-thick AGN: the Circinus galaxy. Probing the geometric arrangement of parsec-scale matter in extragalactic objects is now feasible thanks to X-ray polarimetry.

Multiwavelength Polarization Observations of Mrk 501

Mrk 501 is a prototypical high-synchrotron-peaked blazar and serves as one of the primary targets for the Imaging X-ray Polarimetry Explorer (IXPE). In this study, we report X-ray polarization measurements of Mrk 501 based on six IXPE observations. The detection of X-ray polarization at a confidence level exceeding 99% is achieved in four out of the six observations conducted across the entire energy range (2–8 keV) of IXPE. The maximum polarization degree (ΠX) is measured to be 15.8% ± 2.8%, accompanied by a polarization angle (ψ X) of 98.°0 ± 5.°1 at a confidence level of 5.6σ. During the remaining two observations, only an upper limit of ΠX < 12% could be derived at the 99% confidence level. No temporal variability in polarization is observed throughout all six IXPE observations for Mrk 501. A discernible trend of energy-dependent variation in the polarization degree is detected in optical spectropolarimetry; however, no analogous indication is observed in ΠX. The chromatic behavior of Π and the consistent values of ψ across different frequencies from X-ray to radio bands, along with the agreement between ψ and jet position angle, strongly support the interpretation of the energy-stratified model with shock-accelerated particles in the jet of Mrk 501. Additionally, the possibility of the presence of a global helical magnetic field in the jet of Mrk 501 is discussed.

Unveiling the X-ray polarimetric properties of LMC X−3 with IXPE, NICER, and Swift/XRT

ABSTRACT The incoming Imaging X-ray Polarimetry Explorer (IXPE) observations of X-ray binaries provide a new tool to investigate the underlying accretion geometry. Here, we report the first measurements of X-ray polarization of the extragalactic black-hole X-ray binary LMC X−3. We find a polarization fraction of $\sim 3~{{\ \rm per\ cent}}$ at a polarization angle of ∼135° in the 2–8 keV energy band with statistical significance at the 7σ level. This polarization measurement significantly exceeds the minimum detectable polarization threshold of 1.2 per cent for the source, ascertained at a 99 per cent confidence level within the 2–8 keV energy band. The simultaneous spectro-polarimetric fitting of Neutron Star Interior Composition Explorer, Swift/X-Ray Telescope (XRT), and IXPE revealed the presence of a disc with a temperature of ∼1 keV and a Comptonized component with a power-law index of ∼2.4, confirming the soft nature of the source. The polarization degree increases with energy from ∼3 per cent in the 2–5.7 keV band to ∼9 per cent in the 5.7–8 keV band, while the polarization angle is energy independent. The observed energy dependence and the sudden jump of polarization fraction above 5 keV supports the idea of a static slab coronal geometry for the Comptonizing medium of LMC X−3.

Insights into the broadband emission of the TeV blazar Mrk 501 during the first X-ray polarization measurements

Aims. We present the first multiwavelength study of Mrk 501 that contains simultaneous very-high-energy (VHE) γ-ray observations and X-ray polarization measurements from the Imaging X-ray Polarimetry Explorer (IXPE). Methods. We used radio-to-VHE data from a multiwavelength campaign carried out between March 1, 2022, and July 19, 2022 (MJD 59639 to MJD 59779). The observations were performed by MAGIC, Fermi-LAT, NuSTAR, Swift (XRT and UVOT), and several other instruments that cover the optical and radio bands to complement the IXPE pointings. We characterized the dynamics of the broadband emission around the X-ray polarization measurements through its multiband fractional variability and correlations, and compared changes observed in the polarization degree to changes seen in the broadband emission using a multi-zone leptonic scenario. Results. During the IXPE pointings, the VHE state is close to the average behavior, with a 0.2–1 TeV flux of 20%–50% of the emission of the Crab Nebula. Additionally, it shows low variability and a hint of correlation between VHE γ-rays and X-rays. Despite the average VHE activity, an extreme X-ray behavior is measured for the first two IXPE pointings, taken in March 2022 (MJD 59646 to 59648 and MJD 59665 to 59667), with a synchrotron peak frequency &gt; 1 keV. For the third IXPE pointing, in July 2022 (MJD 59769 to 59772), the synchrotron peak shifts toward lower energies and the optical/X-ray polarization degrees drop. All three IXPE epochs show an atypically low Compton dominance in the γ-rays. The X-ray polarization is systematically higher than at lower energies, suggesting an energy stratification of the jet. While during the IXPE epochs the polarization angles in the X-ray, optical, and radio bands align well, we find a clear discrepancy in the optical and radio polarization angles in the middle of the campaign. Such results further support the hypothesis of an energy-stratified jet. We modeled broadband spectra taken simultaneous to the IXPE pointings, assuming a compact zone that dominates in the X-rays and the VHE band, and an extended zone stretching farther downstream in the jet that dominates the emission at lower energies. NuSTAR data allow us to precisely constrain the synchrotron peak and therefore the underlying electron distribution. The change between the different states observed in the three IXPE pointings can be explained by a change in the magnetization and/or the emission region size, which directly connects the shift in the synchrotron peak to lower energies with the drop in the polarization degree.

First characterization of the emission behavior of Mrk 421 from radio to very high-energy gamma rays with simultaneous X-ray polarization measurements

Aims. We have performed the first broadband study of Mrk 421 from radio to TeV gamma rays with simultaneous measurements of the X-ray polarization from IXPE. Methods. The data were collected as part of an extensive multiwavelength campaign carried out between May and June 2022 using MAGIC, Fermi-LAT, NuSTAR, XMM-Newton, Swift, and several optical and radio telescopes to complement IXPE data. Results. During the IXPE exposures, the measured 0.2–1 TeV flux was close to the quiescent state and ranged from 25% to 50% of the Crab Nebula without intra-night variability. Throughout the campaign, the very high-energy (VHE) and X-ray emission are positively correlated at a 4σ significance level. The IXPE measurements reveal an X-ray polarization degree that is a factor of 2–5 higher than in the optical/radio bands; that implies an energy-stratified jet in which the VHE photons are emitted co-spatially with the X-rays, in the vicinity of a shock front. The June 2022 observations exhibit a rotation of the X-ray polarization angle. Despite no simultaneous VHE coverage being available during a large fraction of the swing, the Swift-XRT monitoring reveals an X-ray flux increase with a clear spectral hardening. This suggests that flares in high synchrotron peaked blazars can be accompanied by a polarization angle rotation, as observed in some flat spectrum radio quasars. Finally, during the polarization angle rotation, NuSTAR data reveal two contiguous spectral hysteresis loops in opposite directions (clockwise and counterclockwise), implying important changes in the particle acceleration efficiency on approximately hour timescales.

First Detection of Polarization in X-Rays for PSR B0540-69 and Its Nebula

We report on X-ray polarization measurements of the extragalactic Crab-like PSR B0540-69 and its Pulsar Wind Nebula (PWN) in the Large Magellanic Cloud, using a ∼850 ks Imaging X-ray Polarimetry Explorer (IXPE) exposure. The PWN is unresolved by IXPE. No statistically significant polarization is detected for the image-averaged data, giving a 99% confidence polarization upper limit (MDP99) of 5.3% in the 2–8 keV energy range. However, a phase-resolved analysis detects polarization for both the nebula and pulsar in the 4–6 keV energy range. For the PWN defined as the off-pulse phases, the polarization degree (PD) of (24.5 ± 5.3)% and polarization angle (PA) of (78.1 ± 6.2)° is detected at 4.6σ significance level, consistent with the PA observed in the optical band. In a single on-pulse window, a hint of polarization is measured at 3.8σ with PD of (50.0 ± 13.1)% and PA of (6.2 ± 7.4)°. A “simultaneous” PSR/PWN analysis finds two bins at the edges of the pulse exceeding 3σ PD significance, with PD of (68 ± 20)% and (62 ± 20)%; intervening bins at 2–3σ significance have lower PD, hinting at additional polarization structure.

Magnetic Structures and Turbulence in SN 1006 Revealed with Imaging X-Ray Polarimetry

Young supernova remnants strongly modify the surrounding magnetic fields, which in turn play an essential role in accelerating cosmic rays (CRs). The X-ray polarization measurements probe magnetic field morphology and turbulence at the immediate acceleration site. We report the X-ray polarization distribution in the northeastern shell of SN 1006 from a 1 Ms observation with the Imaging X-ray Polarimetry Explorer. We found an average polarization degree of 22.4% ± 3.5% and an average polarization angle of −45.°4 ± 4.°5 (measured on the plane of the sky from north to east). The X-ray polarization angle distribution reveals that the magnetic fields immediately behind the shock in the northeastern shell of SN 1006 are nearly parallel to the shock normal or radially distributed, similar to that in the radio observations, and consistent with the quasi-parallel CR acceleration scenario. The X-ray emission is marginally more polarized than that in the radio band. The X-ray polarization degree of SN 1006 is much larger than that in Cas A and Tycho, together with the relatively tenuous and smooth ambient medium of the remnant, favoring that CR-induced instabilities set the magnetic turbulence in SN 1006, and CR acceleration is environment-dependent.

Detection of X-Ray Polarized Emission and Accretion-disk Winds with IXPE and NICER in the Black Hole X-Ray Binary 4U 1630−47

We detect a high level of polarization in the X-ray emission of the black hole binary 4U 1630–47 in an observation with the Imaging X-ray Polarimetry Explorer. The 2–8 keV polarization degree is 8% at a position angle of 18°, with the polarization degree increasing significantly with energy, from ∼6% at ∼2 keV to ∼11% at ∼8 keV. The continuum emission in the spectrum of simultaneous observations with the Neutron Star Interior Composition Explorer (NICER) is well described with only a thermal disk spectrum, with stringent upper limits to any Comptonized emission from the corona. Together with the lack of significant variability in the Fourier power spectrum, this suggests that the source was in the high soft state at the time of these observations. The NICER spectrum reveals the presence of several absorption lines in the 6–9 keV band that we fit with two ionized absorbers, providing evidence of the presence of a strong disk wind, which supports the idea that the source was in the soft state. Previous measurements of X-ray polarization in other sources in harder states were associated with the corona or the jet in those systems. Given that the corona is significantly absent in this observation of 4U 1630–47, and that the jet in black hole binaries is quenched in the high soft state, we speculate that in this observation of 4U 1630–47, the polarization likely arises from the direct and reflected radiation of the accretion disk in this source.

Accretion geometry of the neutron star low mass X-ray binary Cyg X-2 from X-ray polarization measurements

ABSTRACT We report spectro-polarimetric results of an observational campaign of the bright neutron star low-mass X-ray binary Cyg X-2 simultaneously observed by IXPE, NICER, and INTEGRAL. Consistently with previous results, the broad-band spectrum is characterized by a lower-energy component, attributed to the accretion disc with kTin ≈ 1 keV, plus unsaturated Comptonization in thermal plasma with temperature kTe = 3 keV and optical depth τ ≈ 4, assuming a slab geometry. We measure the polarization degree in the 2–8 keV band P = 1.8 ± 0.3 per cent and polarization angle ϕ = 140° ± 4°, consistent with the previous X-ray polarimetric measurements by OSO-8 as well as with the direction of the radio jet which was earlier observed from the source. While polarization of the disc spectral component is poorly constrained with the IXPE data, the Comptonized emission has a polarization degree P = 4.0 ± 0.7 per cent and a polarization angle aligned with the radio jet. Our results strongly favour a spreading layer at the neutron star surface as the main source of the polarization signal. However, we cannot exclude a significant contribution from reflection off the accretion disc, as indicated by the presence of the iron fluorescence line.

Analytical techniques for polarimetric imaging of accretion flows in the Schwarzschild metric

Emission from an accretion disk around compact objects, such as neutron stars and black holes, is expected to be significantly polarized. The polarization can be used to put constraints on the geometrical and physical parameters of the compact sources – their radii, masses, and spins – as well as to determine the orbital parameters. The radiation escaping from the innermost parts of the disk is strongly affected by the gravitational field of the compact object and the relativistic velocities of the matter. The straightforward calculation of the observed polarization signatures involves a computationally expensive ray-tracing technique. At the same time, having fast computational routines for direct data fitting is becoming increasingly important in light of the currently observed images of the accretion flow around the supermassive black hole in M 87 by the Event Horizon Telescope and infrared polarization signatures coming from Sgr A*, as well as the upcoming X-ray polarization measurements by the Imaging X-ray Polarimetry Explorer and enhanced X-ray Timing and Polarimetry mission. In this work, we obtain an exact analytical expression for the rotation angle of the polarization plane in the Schwarzschild metric accounting for the effects of light bending and relativistic aberration. We show that the calculation of the observed flux, polarization degree, and polarization angle as a function of energy can be performed analytically with a high level of accuracy using an approximate light-bending formula, eliminating the need for the precomputed tabular models in fitting routines.

The IXPE instrument calibration equipment

The Imaging X-ray Polarimetry Explorer is a mission dedicated to the measurement of X-ray polarization from tens of astrophysical sources belonging to different classes. Expected to be launched at the end of 2021, the payload comprises three mirrors and three focal plane imaging polarimeters, the latter being designed and built in Italy. While calibration is always an essential phase in the development of high-energy space missions, for IXPE it has been particularly extensive both to calibrate the response to polarization, which is peculiar to IXPE, and to achieve a statistical uncertainty below the expected sensitivity. In this paper we present the calibration equipment that was designed and built at INAF-IAPS in Rome, Italy, for the calibration of the polarization-sensitive focal plane detectors on-board IXPE. Equipment includes calibration sources, both polarized and unpolarized, stages to align and move the beam, test detectors and their mechanical assembly. While all these equipments were designed to fit the specific needs of the IXPE Instrument calibration, their versatility could also be used in the future for other projects.

Oblate Schwarzschild approximation for polarized radiation from rapidly rotating neutron stars

We have developed a complete theory for the calculation of the observed Stokes parameters for radiation emitted from the surface of a rapidly rotating neutron star (NS) using the oblate Schwarzschild approximation. We accounted for the rotation of the polarization plane due to relativistic effects along the path from the stellar surface to the observer. The results were shown to agree with those obtained by performing full numerical general relativistic ray-tracing with the ARCMANCER code. We showed that the obtained polarization angle profiles may differ substantially from those derived for a spherical star. We demonstrated that assuming incorrect shape for the star can lead to biased constraints for NS parameters when fitting the polarization data. Using a simplified model, we also made a rough estimate of how accurately the geometrical parameters of an accreting NS can be determined using the X-ray polarization measurements of upcoming polarimeters like the Imaging X-ray Polarimeter Explorer or the enhanced X-ray Timing and Polarimetry mission.

X-ray Polarization at the Crossroads

AbstractWe argue that measurements of X-ray polarization using the recently launched Imaging X-ray Polarimetry Explorer will answer many open questions about magnetars in particular the physical state of their surfaces, whether vacuum birefringence exists, and the nature of the hard X-ray emission from these objects. We outline the capabilities of the instrument, specific models and the results of simulations for the magnetar 4U 0142+61.

Towards a complete description of spectra and polarization of black hole accretion discs: albedo profiles and returning radiation

ABSTRACT Accretion discs around stellarmass black holes (BHs) emit radiation peaking in the soft X-rays when the source is in the thermal state. The emerging photons are polarized and, for symmetry reasons, the polarization integrated over the source is expected to be either parallel or perpendicular to the (projected) disc symmetry axis, because of electron scattering in the disc. However, due to general relativity effects photon polarization vectors will rotate with respect to their original orientation, by an amount depending on both the BH spin and the observer’s inclination. Hence, X-ray polarization measurements may provide important information about strong gravity effects around these sources. Along with the spectral and polarization properties of radiation which reaches directly the observer once emitted from the disc, in this paper we also include the contribution of returning radiation, i.e. photons that are bent by the strong BH gravity to return again on the disc, where they scatter until eventually escaping to infinity. A comparison between our results and those obtained in previous works by different authors shows an overall good agreement, despite the use of different code architectures. We finally consider the effects of absorption in the disc material by including more realistic albedo profiles for the disc surface. Our findings in this respect show that considering also the ionization state of the disc may deeply modify the behaviour of polarization observables.

Prompt Emission Polarimetry of Gamma-Ray Bursts with the AstroSat CZT Imager

Abstract X-ray and gamma-ray polarization measurements of the prompt emission of gamma-ray bursts (GRBs) are believed to be extremely important for testing various models of GRBs. So far, the available measurements of hard X-ray polarization of GRB prompt emission have not significantly constrained the GRB models, particularly because of the difficulty of measuring polarization in these bands. The CZT Imager (CZTI) on board AstroSat is primarily an X-ray spectroscopic instrument that also works as a wide-angle GRB monitor due to the transparency of its support structure above 100 keV. It also has experimentally verified polarization measurement capability in the energy range 100–300 keV and thus provides a unique opportunity to attempt spectropolarimetric studies of GRBs. Here we present the polarization data for the brightest 11 GRBs detected by CZTI during its first year of operation. Among these, five GRBs show polarization signatures with ⪆3σ, and one GRB shows 2σ detection significance. We place upper limits for the remaining five GRBs. We provide details of the various tests performed to validate our polarization measurements. While it is difficult yet to discriminate between various emission models with the current sample alone, the large number of polarization measurements that CZTI expects to gather in its minimum lifetime of five years should help to significantly improve our understanding of the prompt emission.

Optimization of the gain factors and parameter settings for the new gamma-ray burst polarimeter, POLAR

As a space-borne detector POLAR is designed to conduct hard X-ray polarization measurements of gamma-ray bursts on a statistically significant sample of events and with an unprecedented accuracy. During its development phase a number of tests, calibrations and verification measurements were carried out in order to validate instrument functionality and optimize operational parameters. In this article we present results on gain optimization together with verification data obtained in the course of broad laboratory and environmental tests. In particular we focus on exposures to the 137Cs radioactive source and determination of the gain dependence on the high voltage for all 1600 detection channels of the polarimeter. Performance of the instrument is described in detail with respect to the dynamic range, energy resolution and temperature dependence. Gain optimization algorithms and response non-uniformity studies are also discussed. Results presented below are important for the development of the POLAR calibration and operation database.

Strong higher-order resonant contribution to Fe Kα x-ray line polarization in hot anisotropic plasmas

SynopsisWe report the first systematic measurement of x-ray angular distribution and polarization following dielectronic, trielectronic and quadroelectronic recombination into He-like through O-like iron and krypton ions. The experimental data revealed that the the degree of polarization of x rays is dominated by relativistic Breit interaction and hitherto neglected higher-order trielectronic and quadroelectronic recombination resonances.

Design of the Telescope Truss and Gondola for the Balloon-Borne X-ray Polarimeter X-Calibur

X-ray polarimetry has seen a growing interest in recent years. Improvements in detector technology and focusing X-ray optics now enable sensitive astrophysical X-ray polarization measurements. These measurements will provide new insights into the processes at work in accreting black holes, the emission of X-rays from neutron stars and magnetars, and the structure of AGN jets. X-Calibur is a balloon-borne hard X-ray scattering polarimeter. An X-ray mirror with a focal length of 8[Formula: see text]m focuses X-rays onto the detector, which consists of a plastic scattering element surrounded by Cadmium-Zinc-Telluride detectors, which absorb and record the scattered X-rays. Since X-rays preferentially scatter perpendicular to their polarization direction, the polarization properties of an X-ray beam can be inferred from the azimuthal distribution of scattered X-rays. A close alignment of the X-ray focal spot with the center of the detector is required in order to reduce systematic uncertainties and to maintain a high photon detection efficiency. This places stringent requirements on the mechanical and thermal stability of the telescope structure. During the flight on a stratospheric balloon, X-Calibur makes use of the Wallops Arc-Second Pointer (WASP) to point the telescope at astrophysical sources. In this paper, we describe the design, construction, and test of the telescope structure, as well as its performance during a 25-h flight from Ft. Sumner, New Mexico. The carbon fiber–aluminum composite structure met the requirements set by X-Calibur and its design can easily be adapted for other types of experiments, such as X-ray imaging or spectroscopic telescopes.

Novel octahedral tilt system a + b + c + in (1 − x)Na0.5Bi0.5TiO3–xCdTiO3 solid solutions

(1 − x)Na0.5Bi0.5TiO3–xCdTiO3 solid solutions in the whole concentration range (0.0 ≤ x ≤ 1.0) were studied by means of X-ray diffraction, dielectric spectroscopy and polarization measurements. The study was mainly focused on crystalline structure of the compositions, depending on their place in the phase diagram. The solid solution system exhibits at least four different phases at room temperature, giving rise to paraelectric, ferroelectric and relaxor ferroelectric behaviour. There were proposed appropriate space groups for each of these phases, using Rietveld refinement method for analysis of the X-ray diffraction patterns and taking into account polarization measurement results. Unexpected and unusual octahedral tilt systems—a + a + a + and a + b + c +—were found in certain CdTiO3 concentration ranges. The tilt system a + b + c +, which was detected in the ferroelectric phase, was evidenced for the first time, as it has been theoretically predicted, but never experimentally observed before in any material. It was shown that ferroelectricity in (1 − x)Na0.5Bi0.5TiO3–xCdTiO3 solid solutions arises not only from the Ti+4 displacements, but also from the polar distortions in square planar and cubooctahedral cation A-sites. Upon heating, at a phase transition from the ferroelectric to the paraelectric state, a + b + c + tilt system transforms into a + a + a +. The studied compositions were compared with (1 − x)Na0.5Bi0.5TiO3–xCaTiO3 solid solution system, as CdTiO3 and CaTiO3 are crystallographically very similar. It was revealed that both constituents behave very differently. CaTiO3 in (1 − x)Na0.5Bi0.5TiO3–xCaTiO3, even in low concentrations, stabilizes solid solutions in its Pnma space group, unlike its counterpart CdTiO3 in the studied materials.