| 1. |
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| 2. |
- Heywood, I., et al.
(author)
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Inflation of 430-parsec bipolar radio bubbles in the Galactic Centre by an energetic event
- 2019
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In: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 573:7773, s. 235-237
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Journal article (peer-reviewed)abstract
- The Galactic Centre contains a supermassive black hole with a mass of four million Suns1 within an environment that differs markedly from that of the Galactic disk. Although the black hole is essentially quiescent in the broader context of active galactic nuclei, X-ray observations have provided evidence for energetic outbursts from its surroundings2. Also, although the levels of star formation in the Galactic Centre have been approximately constant over the past few hundred million years, there is evidence of increased short-duration bursts3, strongly influenced by the interaction of the black hole with the enhanced gas density present within the ring-like central molecular zone4 at Galactic longitude |l| < 0.7 degrees and latitude |b| < 0.2 degrees. The inner 200-parsec region is characterized by large amounts of warm molecular gas5, a high cosmic-ray ionization rate6, unusual gas chemistry, enhanced synchrotron emission7,8, and a multitude of radio-emitting magnetized filaments9, the origin of which has not been established. Here we report radio imaging that reveals a bipolar bubble structure, with an overall span of 1 degree by 3 degrees (140 parsecs × 430 parsecs), extending above and below the Galactic plane and apparently associated with the Galactic Centre. The structure is edge-brightened and bounded, with symmetry implying creation by an energetic event in the Galactic Centre. We estimate the age of the bubbles to be a few million years, with a total energy of 7 × 1052 ergs. We postulate that the progenitor event was a major contributor to the increased cosmic-ray density in the Galactic Centre, and is in turn the principal source of the relativistic particles required to power the synchrotron emission of the radio filaments within and in the vicinity of the bubble cavities.
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| 3. |
- Camilo, F., et al.
(author)
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Revival of the Magnetar PSR J1622-4950: Observations with MeerKAT, Parkes, XMM-Newton, Swift, Chandra, and NuSTAR
- 2018
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In: Astrophysical Journal. - : American Astronomical Society. - 1538-4357 .- 0004-637X. ; 856:2
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Journal article (peer-reviewed)abstract
- New radio (MeerKAT and Parkes) and X-ray (XMM-Newton, Swift, Chandra, and NuSTAR) observations of PSR J1622-4950 indicate that the magnetar, in a quiescent state since at least early 2015, reactivated between 2017 March 19 and April 5. The radio flux density, while variable, is approximately 100 larger than during its dormant state. The X-ray flux one month after reactivation was at least 800 larger than during quiescence, and has been decaying exponentially on a 111 19 day timescale. This high-flux state, together with a radio-derived rotational ephemeris, enabled for the first time the detection of X-ray pulsations for this magnetar. At 5%, the 0.3-6 keV pulsed fraction is comparable to the smallest observed for magnetars. The overall pulsar geometry inferred from polarized radio emission appears to be broadly consistent with that determined 6-8 years earlier. However, rotating vector model fits suggest that we are now seeing radio emission from a different location in the magnetosphere than previously. This indicates a novel way in which radio emission from magnetars can differ from that of ordinary pulsars. The torque on the neutron star is varying rapidly and unsteadily, as is common for magnetars following outburst, having changed by a factor of 7 within six months of reactivation.
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| 4. |
- Wiedorn, Max O., et al.
(author)
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Megahertz serial crystallography
- 2018
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In: Nature Communications. - : Nature Publishing Group. - 2041-1723. ; 9
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Journal article (peer-reviewed)abstract
- The new European X-ray Free-Electron Laser is the first X-ray free-electron laser capable of delivering X-ray pulses with a megahertz inter-pulse spacing, more than four orders of magnitude higher than previously possible. However, to date, it has been unclear whether it would indeed be possible to measure high-quality diffraction data at megahertz pulse repetition rates. Here, we show that high-quality structures can indeed be obtained using currently available operating conditions at the European XFEL. We present two complete data sets, one from the well-known model system lysozyme and the other from a so far unknown complex of a beta-lactamase from K. pneumoniae involved in antibiotic resistance. This result opens up megahertz serial femtosecond crystallography (SFX) as a tool for reliable structure determination, substrate screening and the efficient measurement of the evolution and dynamics of molecular structures using megahertz repetition rate pulses available at this new class of X-ray laser source.
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| 5. |
- Ludick, D. J., et al.
(author)
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Enhancing the Jacobi Method with the CBFM for array antenna analysis
- 2017
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In: IEEE Antennas and Propagation Society International Symposium. - 1522-3965. - 9781538632840 ; 2017-January, s. 727-728
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Conference paper (peer-reviewed)abstract
- The analysis of sparse, disjoint finite antenna array structures is considered in this work. The Method-of-Moments (MoM) based CBFM-enhanced Jacobi technique is presented, and offers an improvement over the standard iterative Jacobi method in terms of convergence and accuracy. By applying the Characteristic Basis Function Method (CBFM) during each iteration the effect of mutual coupling between the array elements can be accounted for more accurately than in the standard Jacobi method. The convergence rate of the method is found to be better than that of the Jacobi technique.
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| 6. |
- Botha, J. D. M., et al.
(author)
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Some noise predictions for small wind turbines
- 2016
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In: Proceedings of ISMA 2016 - International Conference on Noise and Vibration Engineering and USD2016 - International Conference on Uncertainty in Structural Dynamics. - : KU Leuven, Departement Werktuigkunde. - 9789073802940 ; , s. 4019-4032
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Conference paper (peer-reviewed)abstract
- In the FP7 SWIP project three small wind turbine designs have been considered. These include two Horizontal Axis machine with a rated power of 4 and 20 kW and a 2 kW Vertical axis configuration. These will be mounted at three pilot sites with supporting atmospheric modelling and measurements. This paper presents broadband aeroacoustic noise predictions. The prediction methods compare semi empirical predictions more usually used for large turbine applications) supported where appropriate by CFD flow calculations. In order to pursue mitigation measures for these scales of machines, the noise is analysed on a component basis and considered from the viewpoints of self noise and inflow noise. The vertical axis machine is shown to dominated by boundary layer noise whereas the horizontal axis machines are more susceptible to inflow noise. In this the modelling of the inflow turbulence spectrum is seen to be critical.
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| 7. |
- Botha, Matthys M, et al.
(author)
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An improved quadrature error estimate for nearly-singular MoM integrals
- 2018
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In: 2018 IEEE Antennas and Propagation Society International Symposium and USNC/URSI National Radio Science Meeting, APSURSI 2018 - Proceedings. ; , s. 2431-2432
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Conference paper (peer-reviewed)abstract
- A well-known numerical integration scheme for weakly near-singular integrands on triangle domains, is the Radial-Angular-RI-Sqrt near-singularity cancellation transformation quadrature scheme. Such integrals feature routinely in the method of moments (MoM), for integral equation based numerical electromagnetic field calculations. Recently, a closed-form quadrature error estimate has been proposed for this scheme. In this paper, the estimate is further improved, such that its range of applicability is extended.
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| 8. |
- Botha, Matthys M, et al.
(author)
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Analysis and estimation of quadrature errors in weakly singular source integrals of the method of moments
- 2018
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In: International Journal of Numerical Modelling: Electronic Networks, Devices and Fields. - : Wiley. - 0894-3370 .- 1099-1204. ; 31:1
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Journal article (peer-reviewed)abstract
- The method of moments (MoM) is used for the numerical solution of electromagnetic field integral equations. Weakly singular integrals over surfaces in 3 dimensions (3D) are routinely evaluated for the impedance matrix setup and for post-processing. Available numerical integration schemes range from direct application of Gauss-Legendre product-rule quadrature, to singularity and near-singularity cancellation, coordinate transformation schemes. This paper presents a general, explicit, pole-based, a priori procedure to estimate quadrature errors in the numerical evaluation of weakly singular and near-singular, 3D surface integrals in the MoM. It is based on an error theorem for linear Gaussian quadrature, which involves the analytic extension of the integrand into the complex plane. Errors are linked to poles in the complex plane. New closed-form estimates are presented for direct Gaussian product-rule integration, polar-coordinate integration, and the Radial-Angular-R 1 -Sqrt singularity cancellation scheme, for triangle integration domains. This work can serve as a foundation/template for further, 3D MoM-related work to identify appropriate quadrature schemes according to their error characteristics; for automatic selection of optimal schemes and quadrature orders in a computer implementation of the MoM; and for local and global estimation of MoM quadrature errors. This work can be specialized to the MoM for surfaces in 2D.
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| 9. |
- Botha, Matthys M, et al.
(author)
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Quadrature error estimation for MoM matrix entries
- 2017
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In: 19th International Conference on Electromagnetics in Advanced Applications, ICEAA 2017; Verona; Italy; 11 September 2017 through 15 September 2017. - 9781509044511 ; , s. 973-975
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Conference paper (peer-reviewed)abstract
- This paper is concerned with the method of moments (MoM) for electric field integral equation (EFIE) based numerical electromagnetic analysis of conducting surface structures. Inner (source) and outer (testing) integrals are encountered, when evaluating matrix entries. The well-known Radial-Angular-R1-Sqrt (RA-R1-Sqrt) weak near singularity cancellation transformation quadrature scheme for the inner integrals and standard Gaussian numerical integration for the outer integrals, are considered. It is shown that the quadrature error in the matrix entries, due to inner integral evaluation, can be accurately estimated under certain circumstances. A closed-form quadrature error estimate for the RA-R1-Sqrt scheme is employed.
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| 10. |
- Galli, Lorenzo, et al.
(author)
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Towards phasing using high X-ray intensity
- 2015
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In: IUCrJ. - 2052-2525. ; 2, s. 627-634
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Journal article (peer-reviewed)abstract
- X-ray free-electron lasers (XFELs) show great promise for macromolecular structure determination from sub-micrometre-sized crystals, using the emerging method of serial femtosecond crystallography. The extreme brightness of the XFEL radiation can multiply ionize most, if not all, atoms in a protein, causing their scattering factors to change during the pulse, with a preferential ‘bleaching’ of heavy atoms. This paper investigates the effects of electronic damage on experimental data collected from a Gd derivative of lysozyme microcrystals at different X-ray intensities, and the degree of ionization of Gd atoms is quantified from phased difference Fourier maps. A pattern sorting scheme is proposed to maximize the ionization contrast and the way in which the local electronic damage can be used for a new experimental phasing method is discussed.
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| 11. |
- Ludick, D. J., et al.
(author)
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Accelerating the CBFM-enhanced jacobi method
- 2017
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In: 19th International Conference on Electromagnetics in Advanced Applications, ICEAA 2017; Verona; Italy; 11 September 2017 through 15 September 2017. - 9781509044511 ; , s. 346-349
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Conference paper (peer-reviewed)abstract
- The Characteristic Basis Function Method (CBFM)-enhanced Jacobi method has been introduced as an improvement to the standard iterative Jacobi method for finite array analysis. This technique is a domain decomposition approach based on the Method of Moments (MoM) formulation. In some cases, e.g. array environments with a low degree of mutual coupling, the runtime benefit of the CBFM-enhanced Jacobi method is not as significant when compared to that of the Jacobi technique. The reason for this is that additional computational overhead is introduced during each iteration, i.e. setting up and solving the CBFM reduced matrix equation. In this work the adaptive cross approximation (ACA) algorithm is used to accelerate this step in the CBFM-enhanced Jacobi method.
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| 12. |
- Ludick, D. J., et al.
(author)
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Comparison of the iterative jacobi method and the iterative Domain Green'S Function Method for finite array analysis
- 2016
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In: 10th European Conference on Antennas and Propagation, EuCAP 2016, Davos, Switzerland, 10-15 April 2016. - 2164-3342. - 9788890701863
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Conference paper (peer-reviewed)abstract
- The purpose of this work is to compare two iterative techniques that may be used for the analysis of large, disjoint finite antenna arrays, viz. the iterative Jacobi method and the iterative Domain Green's Function Method. These methods are conceptually similar, in that they offer alternative ways to improve non-local current distributions during the iterative process. The error convergence of each method will be studied at the hand of an example.
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| 13. |
- Nass, Karol, et al.
(author)
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Indications of radiation damage in ferredoxin microcrystals using high-intensity X-FEL beams
- 2015
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In: Journal of Synchrotron Radiation. - 0909-0495 .- 1600-5775. ; 22:2, s. 225-238
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Journal article (peer-reviewed)abstract
- Proteins that contain metal cofactors are expected to be highly radiation sensitive since the degree of X-ray absorption correlates with the presence of high-atomic-number elements and X-ray energy. To explore the effects of local damage in serial femtosecond crystallography (SFX), Clostridium ferredoxin was used as a model system. The protein contains two [4Fe–4S] clusters that serve as sensitive probes for radiation-induced electronic and structural changes. High-dose room-temperature SFX datasets were collected at the Linac Coherent Light Source of ferredoxin microcrystals. Difference electron density maps calculated from high-dose SFX and synchrotron data show peaks at the iron positions of the clusters, indicative of decrease of atomic scattering factors due to ionization. The electron density of the two [4Fe–4S] clusters differs in the FEL data, but not in the synchrotron data. Since the clusters differ in their detailed architecture, this observation is suggestive of an influence of the molecular bonding and geometry on the atomic displacement dynamics following initial photoionization. The experiments are complemented by plasma code calculations.
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| 14. |
- Rasam, Amin, et al.
(author)
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Aerodynamic noise prediction for a wind turbine using numerical flow simulations and semi-empirical modelling approaches
- 2016
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In: 22nd AIAA/CEAS Aeroacoustics Conference. - Reston, Virginia : American Institute of Aeronautics and Astronautics.
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Conference paper (other academic/artistic)abstract
- In this paper, aerodynamic and aero-acoustic simulations are performed for a small horizontal axis wind turbine, suitable for the integration of wind energy in urban and peri-urban areas. Detached-eddy simulation (DES) of compressible flow is performed to compute the flow field over the wind turbine. The far-field noise is computed using the Ffowcs - Williams and Hawkings acoustic analogy. Furthermore, the blade element momentum theory is used with a semi-empirical acoustic modeling approach to predict the wind turbine noise. The acoustic modeling approach is based on a semi-empirical formulation for airfoil self noise and an analytic formulation for inflow noise.
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