Ultra-high Angular Resolution Research Articles

A method with ultra-high angular resolution for X-ray diffraction experiments.

In X-ray diffraction measurements, the angular resolution has a detection limit due to the receiving size of the detector. In many cases this detection limit is too large and must be breached to obtain the desired information. A novel method is proposed here by making the detector simultaneously measuring and moving. Using the deconvolution algorithm to remove the convolution effect, the pixel size limitation is finally broken. The algorithm used is not a common one, and suppresses signals at high frequencies, ensuring the reliability of the peak shape after restoration. The feasibility of this method is verified by successfully measuring the crystal truncation rod signal of SrTiO3 single crystal, and the resolution is nearly ten times higher than that of a single pixel. Moreover, this method greatly reduces the noise and improves the signal-to-noise ratio.

An open resource combining multi-contrast MRI and microscopy in the macaque brain

Understanding brain structure and function often requires combining data across different modalities and scales to link microscale cellular structures to macroscale features of whole brain organisation. Here we introduce the BigMac dataset, a resource combining in vivo MRI, extensive postmortem MRI and multi-contrast microscopy for multimodal characterisation of a single whole macaque brain. The data spans modalities (MRI and microscopy), tissue states (in vivo and postmortem), and four orders of spatial magnitude, from microscopy images with micrometre or sub-micrometre resolution, to MRI signals on the order of millimetres. Crucially, the MRI and microscopy images are carefully co-registered together to facilitate quantitative multimodal analyses. Here we detail the acquisition, curation, and first release of the data, that together make BigMac a unique, openly-disseminated resource available to researchers worldwide. Further, we demonstrate example analyses and opportunities afforded by the data, including improvement of connectivity estimates from ultra-high angular resolution diffusion MRI, neuroanatomical insight provided by polarised light imaging and myelin-stained histology, and the joint analysis of MRI and microscopy data for reconstruction of the microscopy-inspired connectome. All data and code are made openly available.

Quantitative Analysis of Dislocations in 4H-SiC Wafers Using Synchrotron X-Ray Topography with Ultra-High Angular Resolution

The wide band gap semiconductor, 4H-SiC, is a promising material for high-power and high-temperature devices due to its outstanding physical properties. Unlike Silicon, defects in 4H-SiC still cannot be completely eliminated even using the latest crystal growth techniques. These defects can be detrimental to the devices and it is important to investigate their properties. The non-destructive characterization technique, X-ray topography (XRT), has been developed over several decades and widely applied to study the crystallographic features of various materials. The utilization of a Si (331) beam conditioner together with a Si (111) double-crystal monochromator (DCM) enables the angular resolution of XRT to be increased by an order of magnitude compared to grazing incidence topography (GIT) or back reflection topography conducted with the DCM alone. This improved technique with extremely small beam divergence is referred to as synchrotron X-ray plane-wave topography (SXPWT). In this study, we will show that the rocking curve width of 4H-SiC 0008 in PWT is only 2.5ʹʹ and thus the lattice distortion at the scale of 1ʹʹ will significantly affect the diffracted intensity. Herein, we report the ultra-high angular resolution in SXPWT enables the detailed probe of the lattice distortion outside the dislocation core in 4H-SiC where the sign of the Burgers vector can be readily determined (Fig. 1). Additionally, dynamical theory of X-ray diffraction was applied to calculate the rocking curve (reflectivity) in SXPWT, and the ray tracing simulation was modified accordingly to calculate the dislocation images under any diffraction conditions. Figure 1

Quantitative analysis of dislocations in 4H-SiC wafers using synchrotron X-ray topography with ultra-high angular resolution

Utilization of an Si(331) beam conditioner together with an Si(111) double-crystal monochromator (DCM) enables the angular resolution of synchrotron X-ray topography to be increased by an order of magnitude compared with grazing-incidence topography or back-reflection topography conducted with the DCM alone. This improved technique with extremely small beam divergence is referred to as synchrotron X-ray plane-wave topography (SXPWT). This study demonstrates that the rocking curve width of 4H-SiC 0008 in PWT is only 2.5′′ and thus the lattice distortion at the scale of 1′′ will significantly affect the diffracted intensity. This work reports the ultra-high angular resolution in SXPWT which enables detailed probing of the lattice distortion outside the dislocation core in 4H-SiC, where the sign of the Burgers vector can be readily determined through comparison with ray-tracing simulations.

The science case and challenges of space-borne sub-millimeter interferometry

Ultra-high angular resolution in astronomy has always been an important vehicle for making fundamental discoveries. Recent results in direct imaging of the vicinity of the supermassive black hole in the nucleus of the radio galaxy M87 by the millimeter VLBI system Event Horizon Telescope and various pioneering results of the Space VLBI mission RadioAstron provided new momentum in high angular resolution astrophysics. In both mentioned cases, the angular resolution reached the values of about 10–20 microarcseconds (0.05–0.1 nanoradian). Further developments towards at least an order of magnitude “sharper” values, at the level of 1 microarcsecond are dictated by the needs of advanced astrophysical studies. The paper emphasis that these higher values can only be achieved by placing millimeter and submillimeter wavelength interferometric systems in space. A concept of such the system, called Terahertz Exploration and Zooming-in for Astrophysics, has been proposed in the framework of the ESA Call for White Papers for the Voyage 2050 long term plan in 2019. In the current paper we present new science objectives for such the concept based on recent results in studies of active galactic nuclei and supermassive black holes. We also discuss several approaches for addressing technological challenges of creating a millimeter/sub-millimeter wavelength interferometric system in space. In particular, we consider a novel configuration of a space-borne millimeter/sub-millimeter antenna which might resolve several bottlenecks in creating large precise mechanical structures. The paper also presents an overview of prospective space-qualified technologies of low-noise analogue front-end instrumentation for millimeter/sub-millimeter telescopes. Data handling and processing instrumentation is another key technological component of a sub-millimeter Space VLBI system. Requirements and possible implementation options for this instrumentation are described as an extrapolation of the current state-of-the-art Earth-based VLBI data transport and processing instrumentation. The paper also briefly discusses approaches to the interferometric baseline state vector determination and synchronisation and heterodyning system. The technology-oriented sections of the paper do not aim at presenting a complete set of technological solutions for sub-millimeter (terahertz) space-borne interferometers. Rather, in combination with the original ESA Voyage 2050 White Paper, it sharpens the case for the next generation microarcsecond-level imaging instruments and provides starting points for further in-depth technology trade-off studies.

Event Horizon Telescope imaging of the archetypal blazar 3C 279 at an extreme 20 microarcsecond resolution

3C 279 is an archetypal blazar with a prominent radio jet that show broadband flux density variability across the entire electromagnetic spectrum. We use an ultra-high angular resolution technique – global Very Long Baseline Interferometry (VLBI) at 1.3 mm (230 GHz) – to resolve the innermost jet of 3C 279 in order to study its fine-scale morphology close to the jet base where highly variableγ-ray emission is thought to originate, according to various models. The source was observed during four days in April 2017 with the Event Horizon Telescope at 230 GHz, including the phased Atacama Large Millimeter/submillimeter Array (ALMA), at an angular resolution of ∼20 μas (at a redshift ofz = 0.536 this corresponds to ∼0.13 pc ∼ 1700 Schwarzschild radii with a black hole massMBH = 8 × 108 M⊙). Imaging and model-fitting techniques were applied to the data to parameterize the fine-scale source structure and its variation. We find a multicomponent inner jet morphology with the northernmost component elongated perpendicular to the direction of the jet, as imaged at longer wavelengths. The elongated nuclear structure is consistent on all four observing days and across different imaging methods and model-fitting techniques, and therefore appears robust. Owing to its compactness and brightness, we associate the northern nuclear structure as the VLBI “core”. This morphology can be interpreted as either a broad resolved jet base or a spatially bent jet. We also find significant day-to-day variations in the closure phases, which appear most pronounced on the triangles with the longest baselines. Our analysis shows that this variation is related to a systematic change of the source structure. Two inner jet components move non-radially at apparent speeds of ∼15 cand ∼20 c(∼1.3 and ∼1.7 μas day−1, respectively), which more strongly supports the scenario of traveling shocks or instabilities in a bent, possibly rotating jet. The observed apparent speeds are also coincident with the 3C 279 large-scale jet kinematics observed at longer (cm) wavelengths, suggesting no significant jet acceleration between the 1.3 mm core and the outer jet. The intrinsic brightness temperature of the jet components are ≲1010K, a magnitude or more lower than typical values seen at ≥7 mm wavelengths. The low brightness temperature and morphological complexity suggest that the core region of 3C 279 becomes optically thin at short (mm) wavelengths.

Revealing the Hippocampal Connectome through Super-Resolution 1150-Direction Diffusion MRI

The hippocampus is a key component of emotional and memory circuits and is broadly connected throughout the brain. We tracked the whole-brain connections of white matter fibres from the hippocampus using ultra-high angular resolution diffusion MRI in both a single 1150-direction dataset and a large normal cohort (n = 94; 391-directions). Using a connectomic approach, we identified six dominant pathways in terms of strength, length and anatomy, and characterised them by their age and gender variation. The strongest individual connection was to the ipsilateral thalamus. There was a strong age dependence of hippocampal connectivity to medial occipital regions. Overall, our results concur with preclinical and ex-vivo data, confirming that meaningful in vivo characterisation of hippocampal connections is possible in an individual. Our findings extend the collective knowledge of hippocampal anatomy, highlighting the importance of the spinal-limbic pathway and the striking lack of hippocampal connectivity with motor and sensory cortices.

Spatially resolved origin of millimeter-wave linear polarization in the nuclear region of 3C 84

We report results from a deep polarization imaging of the nearby radio galaxy 3C 84 (NGC 1275). The source was observed with the Global Millimeter VLBI Array (GMVA) at 86 GHz at an ultrahigh angular resolution of 50 μas (corresponding to ∼200Rs). We also add complementary multiwavelength data from the Very Long Baseline Array (VLBA; 15 and 43 GHz) and from the Atacama Large Millimeter/submillimeter Array (ALMA; 97.5, 233.0 and 343.5 GHz). At 86 GHz, we measured a fractional linear polarization of ∼2% in the VLBI core region. The polarization morphology suggests that the emission is associated with an underlying limb-brightened jet. The fractional linear polarization is lower at 43 and 15 GHz (∼0.3−0.7% and <0.1%, respectively). This suggests an increasing linear polarization degree toward shorter wavelengths on VLBI scales. We also obtain a large rotation measure (RM) of ∼105–6 rad m2in the core at ≳43 GHz. Moreover, the VLBA 43 GHz observations show a variable RM in the VLBI core region during a small flare in 2015. Faraday depolarization and Faraday conversion in an inhomogeneous and mildly relativistic plasma could explain the observed linear polarization characteristics and the previously measured frequency dependence of the circular polarization. Our Faraday depolarization modeling suggests that the RM most likely originates from an external screen with a highly uniform RM distribution. To explain the large RM value, the uniform RM distribution and the RM variability, we suggest that the Faraday rotation is caused by a boundary layer in a transversely stratified jet. Based on the RM and the synchrotron spectrum of the core, we provide an estimate for the magnetic field strength and the electron density of the jet plasma.

The ALMA Phasing System: A Beamforming Capability for Ultra-high-resolution Science at (Sub)Millimeter Wavelengths

The Atacama Millimeter/submillimeter Array (ALMA) Phasing Project (APP) has developed and deployed the hardware and software necessary to coherently sum the signals of individual ALMA antennas and record the aggregate sum in Very Long Baseline Interferometry (VLBI) Data Exchange Format. These beamforming capabilities allow the ALMA array to collectively function as the equivalent of a single large aperture and participate in global VLBI arrays. The inclusion of phased ALMA in current VLBI networks operating at (sub)millimeter wavelengths provides an order of magnitude improvement in sensitivity, as well as enhancements in u–v coverage and north–south angular resolution. The availability of a phased ALMA enables a wide range of new ultra-high angular resolution science applications, including the resolution of supermassive black holes on event horizon scales and studies of the launch and collimation of astrophysical jets. It also provides a high-sensitivity aperture that may be used for investigations such as pulsar searches at high frequencies. This paper provides an overview of the ALMA Phasing System design, implementation, and performance characteristics.

PKS 1502+106: A high-redshiftFermiblazar at extreme angular resolution

Context. Blazars are among the most energetic objects in the Universe. In 2008 August, Fermi/LAT detected the blazar PKS 1502+106 showing a rapid and strong gamma-ray outburst followed by high and variable flux over the next months. This activity at high energies triggered an intensive multi-wavelength campaign covering also the radio, optical, UV, and X-ray bands indicating that the flare was accompanied by a simultaneous outburst at optical/UV/X-rays and a delayed outburst at radio bands. Aims: In the current work we explore the phenomenology and physical conditions within the ultra-relativistic jet of the gamma-ray blazar PKS 1502+106. Additionally, we address the question of the spatial localization of the MeV/GeV-emitting region of the source. Methods: We utilize ultra-high angular resolution mm-VLBI observations at 43 and 86 GHz complemented by VLBI observations at 15 GHz. We also employ single-dish radio data from the F-GAMMA program at frequencies matching the VLBI monitoring. Results: PKS 1502+106 shows a compact core-jet morphology and fast superluminal motion with apparent speeds in the range 5--22 c. Estimation of Doppler factors along the jet yield values between ~7 up to ~50. This Doppler factor gradient implies an accelerating jet. The viewing angle towards the source differs between the inner and outer jet, with the former at ~3 degrees and the latter at ~1 degree, after the jet bends towards the observer beyond 1 mas. The de-projected opening angle of the ultra-fast, magnetically-dominated jet is found to be (3.8 +/- 0.5) degrees. A single jet component can be associated with the pronounced flare both at high-energies and in radio bands. Finally, the gamma-ray emission region is localized at less than 5.9 pc away from the jet base.

Radioastron: Main results of the implementation of the early science program in studies of astronomical objects in the universe with ultra-high angular resolution

The paper presents the main results of the implementation of the Radioastron Early Science Program. Interferometric responses (fringes) were obtained for all types of studied radio sources (quasars, pulsars and cosmic masers) and in all ranges of wavelengths (from meter to centimeter range) with large spaceground baselines. Such measurements have provided a record angular resolution, in some cases reaching several tens of microseconds of arc. This brings unique scientific results concerning the nature of the processes occurring in the vicinity of the supermassive black holes, the structure of the interstellar plasma inhomogeneities and dynamics of compact objects in star-forming regions.

A MULTI-WAVELENGTH POLARIMETRIC STUDY OF THE BLAZAR CTA 102 DURING A GAMMA-RAY FLARE IN 2012

We perform a multi-wavelength polarimetric study of the quasar CTA 102 during an extraordinarily bright $\gamma$-ray outburst detected by the {\it Fermi} Large Area Telescope in September-October 2012 when the source reached a flux of F$_{>100~\mathrm{MeV}} =5.2\pm0.4\times10^{-6}$ photons cm$^{-2}$ s$^{-1}$. At the same time the source displayed an unprecedented optical and NIR outburst. We study the evolution of the parsec scale jet with ultra-high angular resolution through a sequence of 80 total and polarized intensity Very Long Baseline Array images at 43 GHz, covering the observing period from June 2007 to June 2014. We find that the $\gamma$-ray outburst is coincident with flares at all the other frequencies and is related to the passage of a new superluminal knot through the radio core. The powerful $\gamma$-ray emission is associated with a change in direction of the jet, which became oriented more closely to our line of sight ($\theta\sim$1.2$^{\circ}$) during the ejection of the knot and the $\gamma$-ray outburst. During the flare, the optical polarized emission displays intra-day variability and a clear clockwise rotation of EVPAs, which we associate with the path followed by the knot as it moves along helical magnetic field lines, although a random walk of the EVPA caused by a turbulent magnetic field cannot be ruled out. We locate the $\gamma$-ray outburst a short distance downstream of the radio core, parsecs from the black hole. This suggests that synchrotron self-Compton scattering of near-infrared to ultraviolet photons is the probable mechanism for the $\gamma$-ray production.

High Resolution Magnetometer Based on a High Frequency Magnetoelectric MEMS-CMOS Oscillator

This paper demonstrates a miniaturized and high resolution (16 nT/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> ) magnetometer based on a high frequency (168.1 MHz) magnetoelectric Microelectromechanical Systems-Complementary metal-oxidesemiconductor (MEMSCMOS) oscillator. For the first time, a high frequency and high electromechanical performance (quality factor, Q ~ 1084 and electromechanical coupling coefficient, k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ~ 1.18%) magnetoelectric micromechanical resonator based on a self-biased aluminum nitride/iron-gallium-boron (AlN/FeGaB) bilayer nanoplate (250/250 nm) is implemented and used to synthesize a low noise frequency source (2.7 Hz/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> ) whose output frequency is highly sensitive to external magnetic field (169 Hz/μT at zero magnetic field bias). The angular sensitivity of the magnetometer for electronic compass applications is also investigated showing an ultrahigh angular resolution of 0.34° for a 10-μT conservative estimate of the earth's magnetic field, due to the strongly anisotropic sensitivity of the self-biased AlN/FeGaB magnetoelectric resonator. This paper represents the first demonstration of a high resolution self-biased MEMS magnetoelectric resonant sensor interfaced to a compact and low power self-sustained CMOS oscillator as direct frequency readout for the implementation of miniaturized and low power magnetometers with detection limit pushed in ~10s nT/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> range.

The Chinese space Millimeter-wavelength VLBI array—A step toward imaging the most compact astronomical objects

The Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences (CAS) is studying a space VLBI (Very Long Baseline Interferometer) program. The ultimate objective of the program is to image the immediate vicinity of the supermassive black holes (SMBHs) in the hearts of galaxies with a space-based VLBI array working at sub-millimeter wavelengths and to gain ultrahigh angular resolution. To achieve this ambitious goal, the mission plan is divided into three stages. The first phase of the program is called the Space Millimeter-wavelength VLBI Array (SMVA) consisting of two satellites, each carrying a 10-m diameter radio telescope into elliptical orbits with an apogee height of 60,000km and a perigee height of 1200km. The VLBI telescopes in space will work at three frequency bands, 43, 22 and 8GHz. The 43- and 22-GHz bands will be equipped with cryogenic receivers. The space telescopes, observing together with ground-based radio telescopes, enable the highest angular resolution of 20μarcsec at 43GHz. The SMVA is expected to conduct a broad range of high-resolution research, e.g. imaging the shadow of (dark region surrounding) the supermassive black hole in the heart of the galaxy M87 for the first time, studying the kinematics of water megamasers surrounding the SMBHs, and exploring the power source of active galactic nuclei. Pre-research funding has been granted by the CAS in October 2012, to support scientific and technical feasibility studies. These studies also include the manufacturing of a prototype of the deployable 10-m space-based telescope and a 22-GHz receiver. Here we report on the latest progress of the SMVA project.

Dynamical Line-Shifts in High-Field Electron Spin Resonance: Applications to Polymer Physics

Abstract High-field high-frequency Electron Paramagnetic Resonance spectroscopy (HF 2 -EPR) is a powerful tool to investigate, with ultra-high angular resolution, the rotational dynamics of complex systems like polymers, viscous fluids and glasses. Usually, information is drawn by detailed numerical analysis of the overall lineshape. Here, we present a simplified analytical model of the line shifts due to the rotational dynamics of the paramagnetic centre. The model captures the basic features of the reorientation process (time scale and size of the angular jump). It is compared with experimental results concerning the reorientation of a paramagnetic guest molecule dissolved in polystyrene. We find that, if the rotational model to describe the reorientation of the radical is consistent, the best-fit parameters yield equally acceptable best-fits of the overall spectrum by numerical simulations and dynamical line shifts by independent analytic expressions.

Remarkable precision of the 90-year-old dynamic diffraction theories of Darwin and Ewald

The reflectivity functions calculated by Bonse and Hart for a multi-bounce channel-cut single crystal, following the 90-year-old theories of Darwin and Ewald, exhibit extremely narrow nearly rectangular profiles. This feature provides ultra-high angular resolution and sensitivity for the double-crystal diffractometers now widely used for the observation of large-scale structures in condensed matter. However, the experimental results are several orders of magnitude poorer than the theoretical prediction, the `wings problem'. The reason for this discrepancy has remained unidentified for more than 40 years, creating problems for both theory and practice. A solution to this problem for neutron diffraction in Si channel-cut crystals is presented here. The results enable nearly theoretical functions of multiple reflectivity to be obtained, demonstrate the remarkable precision of the Darwin and Ewald theories in the range of the wings, and give rise to much improved sensitivity for the next generation of Bonse–Hart double-crystal diffractometers.

X-ray interferometry with transmissive beam combiners for ultra-high angular resolution astronomy

Interferometry provides one of the possible routes to ultra-high angular resolution for X-ray and gamma-ray astronomy. Sub-micro-arc-second angular resolution, necessary to achieve objectives such as imaging the regions around the event horizon of a super-massive black hole at the center of an active galaxy, can be achieved if beams from parts of the incoming wavefront separated by 100s of meters can be stably and accurately brought together at small angles. One way of achieving this is by using grazing incidence mirrors. We here investigate an alternative approach in which the beams are recombined by optical elements working in transmission. It is shown that the use of diffractive elements is a particularly attractive option. We report experimental results from a simple 2-beam interferometer using a low-cost commercially available profiled film as the diffractive elements. A rotationally symmetric filled (or mostly filled) aperture variant of such an interferometer, equivalent to an X-ray axicon, is shown to offer a much wider bandpass than either a Phase Fresnel Lens (PFL) or a PFL with a refractive lens in an achromatic pair. Simulations of an example system are presented.

Efficient Fresnel x-ray optics made simple

A practical design for upcoming spaceborne x-ray telescopes with ultrahigh angular resolution is proposed. Particular attention is directed to technological simplicity and robust as well as cheap components. Based on dispersion corrected Fresnel lenses, an optimized arrangement will be identified with respect to the instrumental sensitivity for a given focal spot size. We show that this optical Gamow peak essentially depends on the radial transmission profile of a diffractive-refractive aperture. Examples for energies above 4 keV illustrate astronomical capabilities for large-scale compact and segmented objectives as well. The spectral and spatial resolutions of conventional semiconductor detectors are very well matched to imaging characteristics of those achromatic lenses. The constraints to fabrication techniques using most promising materials like Li, Be, and plastics are discussed.

Future Prospects for Ultra-High Resolution Imaging of Binary Systems at UV and X-ray Wavelengths

UV and X-ray space-based interferometry will open unprecedented possibilities for spectral and spatial studies of a wide range of currently unresolvable interacting systems. Ultra-high angular resolution direct imaging of individual components and transport processes in interacting binary systems is essential for detailed studies and modeling of accretion and activity. Understanding the mass loss characteristics of both components, and the dynamics of the system as a function of time, will provide key inputs to evolutionary models and will revolutionize our view and understanding of the Universe.

A solution to the problem of phaseless mapping for a high-orbit space—Ground radio interferometer

We consider the problem of mapping with ultra-high angular resolution using a space—ground radio interferometer with a space antenna in a high orbit, whose apogee height exceeds the radius of the Earth by a factor of ten. In this case, a multielement interferometer essentially degenerates into a two-element interferometer. The degeneracy of the close-phase relations prevents the use of standard methods for hybrid mapping and self-calibration for the correct reconstruction of images. We propose a new phaseless mapping method based on methods for the reconstruction of images in the complete absence of phase information, using only the amplitudes of the spatial-coherence function of the source. In connection with this problem, we propose a new method for the reliable solution of the phase problem, based on optimizing information-carrying nonlinear functionals, in particular, the Shannon entropy. Results of simulations of mapping radio sources with various structures with ultra-high angular resolution in the framework of the RADIOASTRON mission are presented.