| 1. |
- Akiyama, Kazunori, et al.
(författare)
-
First Sagittarius A* Event Horizon Telescope Results. II. EHT and Multiwavelength Observations, Data Processing, and Calibration
- 2022
-
Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 930:2
-
Tidskriftsartikel (refereegranskat)abstract
- We present Event Horizon Telescope (EHT) 1.3 mm measurements of the radio source located at the position of the supermassive black hole Sagittarius A* (Sgr A*), collected during the 2017 April 5-11 campaign. The observations were carried out with eight facilities at six locations across the globe. Novel calibration methods are employed to account for Sgr A*'s flux variability. The majority of the 1.3 mm emission arises from horizon scales, where intrinsic structural source variability is detected on timescales of minutes to hours. The effects of interstellar scattering on the image and its variability are found to be subdominant to intrinsic source structure. The calibrated visibility amplitudes, particularly the locations of the visibility minima, are broadly consistent with a blurred ring with a diameter of similar to 50 mu as, as determined in later works in this series. Contemporaneous multiwavelength monitoring of Sgr A* was performed at 22, 43, and 86 GHz and at near-infrared and X-ray wavelengths. Several X-ray flares from Sgr A* are detected by Chandra, one at low significance jointly with Swift on 2017 April 7 and the other at higher significance jointly with NuSTAR on 2017 April 11. The brighter April 11 flare is not observed simultaneously by the EHT but is followed by a significant increase in millimeter flux variability immediately after the X-ray outburst, indicating a likely connection in the emission physics near the event horizon. We compare Sgr A*'s broadband flux during the EHT campaign to its historical spectral energy distribution and find that both the quiescent emission and flare emission are consistent with its long-term behavior.
|
|
| 2. |
- Akiyama, Kazunori, et al.
(författare)
-
First Sagittarius A* Event Horizon Telescope Results. III. Imaging of the Galactic Center Supermassive Black Hole
- 2022
-
Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 930:2
-
Tidskriftsartikel (refereegranskat)abstract
- We present the first event-horizon-scale images and spatiotemporal analysis of Sgr A* taken with the Event Horizon Telescope in 2017 April at a wavelength of 1.3 mm. Imaging of Sgr A* has been conducted through surveys over a wide range of imaging assumptions using the classical CLEAN algorithm, regularized maximum likelihood methods, and a Bayesian posterior sampling method. Different prescriptions have been used to account for scattering effects by the interstellar medium toward the Galactic center. Mitigation of the rapid intraday variability that characterizes Sgr A* has been carried out through the addition of a "variability noise budget" in the observed visibilities, facilitating the reconstruction of static full-track images. Our static reconstructions of Sgr A* can be clustered into four representative morphologies that correspond to ring images with three different azimuthal brightness distributions and a small cluster that contains diverse nonring morphologies. Based on our extensive analysis of the effects of sparse (u, v)-coverage, source variability, and interstellar scattering, as well as studies of simulated visibility data, we conclude that the Event Horizon Telescope Sgr A* data show compelling evidence for an image that is dominated by a bright ring of emission with a ring diameter of similar to 50 mu as, consistent with the expected "shadow" of a 4 x 10(6) M (circle dot) black hole in the Galactic center located at a distance of 8 kpc.
|
|
| 3. |
- Akiyama, Kazunori, et al.
(författare)
-
First Sagittarius A* Event Horizon Telescope Results. IV. Variability, Morphology, and Black Hole Mass
- 2022
-
Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 930:2
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper we quantify the temporal variability and image morphology of the horizon-scale emission from Sgr A*, as observed by the EHT in 2017 April at a wavelength of 1.3 mm. We find that the Sgr A* data exhibit variability that exceeds what can be explained by the uncertainties in the data or by the effects of interstellar scattering. The magnitude of this variability can be a substantial fraction of the correlated flux density, reaching similar to 100% on some baselines. Through an exploration of simple geometric source models, we demonstrate that ring-like morphologies provide better fits to the Sgr A* data than do other morphologies with comparable complexity. We develop two strategies for fitting static geometric ring models to the time-variable Sgr A* data; one strategy fits models to short segments of data over which the source is static and averages these independent fits, while the other fits models to the full data set using a parametric model for the structural variability power spectrum around the average source structure. Both geometric modeling and image-domain feature extraction techniques determine the ring diameter to be 51.8 +/- 2.3 mu as (68% credible intervals), with the ring thickness constrained to have an FWHM between similar to 30% and 50% of the ring diameter. To bring the diameter measurements to a common physical scale, we calibrate them using synthetic data generated from GRMHD simulations. This calibration constrains the angular size of the gravitational radius to be 4.8(-0.7)(+1.4) mu as, which we combine with an independent distance measurement from maser parallaxes to determine the mass of Sgr A* to be 4.0(-0.6)(+1.1) x 10(6) M-circle dot.
|
|
| 4. |
- Akiyama, Kazunori, et al.
(författare)
-
First Sagittarius A* Event Horizon Telescope Results. VI. Testing the Black Hole Metric
- 2022
-
Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 930:2
-
Tidskriftsartikel (refereegranskat)abstract
- Astrophysical black holes are expected to be described by the Kerr metric. This is the only stationary, vacuum, axisymmetric metric, without electromagnetic charge, that satisfies Einstein's equations and does not have pathologies outside of the event horizon. We present new constraints on potential deviations from the Kerr prediction based on 2017 EHT observations of Sagittarius A* (Sgr A*). We calibrate the relationship between the geometrically defined black hole shadow and the observed size of the ring-like images using a library that includes both Kerr and non-Kerr simulations. We use the exquisite prior constraints on the mass-to-distance ratio for Sgr A* to show that the observed image size is within similar to 10% of the Kerr predictions. We use these bounds to constrain metrics that are parametrically different from Kerr, as well as the charges of several known spacetimes. To consider alternatives to the presence of an event horizon, we explore the possibility that Sgr A* is a compact object with a surface that either absorbs and thermally reemits incident radiation or partially reflects it. Using the observed image size and the broadband spectrum of Sgr A*, we conclude that a thermal surface can be ruled out and a fully reflective one is unlikely. We compare our results to the broader landscape of gravitational tests. Together with the bounds found for stellar-mass black holes and the M87 black hole, our observations provide further support that the external spacetimes of all black holes are described by the Kerr metric, independent of their mass.
|
|
| 5. |
- Broderick, Avery E., et al.
(författare)
-
Characterizing and Mitigating Intraday Variability: Reconstructing Source Structure in Accreting Black Holes with mm-VLBI
- 2022
-
Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 930:2
-
Tidskriftsartikel (refereegranskat)abstract
- The extraordinary physical resolution afforded by the Event Horizon Telescope has opened a window onto the astrophysical phenomena unfolding on horizon scales in two known black holes, M87* and Sgr A*. However, with this leap in resolution has come a new set of practical complications. Sgr A* exhibits intraday variability that violates the assumptions underlying Earth aperture synthesis, limiting traditional image reconstruction methods to short timescales and data sets with very sparse (u, v) coverage. We present a new set of tools to detect and mitigate this variability. We develop a data-driven, model-agnostic procedure to detect and characterize the spatial structure of intraday variability. This method is calibrated against a large set of mock data sets, producing an empirical estimator of the spatial power spectrum of the brightness fluctuations. We present a novel Bayesian noise modeling algorithm that simultaneously reconstructs an average image and statistical measure of the fluctuations about it using a parameterized form for the excess variance in the complex visibilities not otherwise explained by the statistical errors. These methods are validated using a variety of simulated data, including general relativistic magnetohydrodynamic simulations appropriate for Sgr A* and M87*. We find that the reconstructed source structure and variability are robust to changes in the underlying image model. We apply these methods to the 2017 EHT observations of M87*, finding evidence for variability across the EHT observing campaign. The variability mitigation strategies presented are widely applicable to very long baseline interferometry observations of variable sources generally, for which they provide a data-informed averaging procedure and natural characterization of inter-epoch image consistency.
|
|
| 6. |
- Sun, Wenbo, et al.
(författare)
-
Quantifying H&E staining results, grading and predicting IDH mutation status of gliomas using hybrid multi-dimensional MRI
-
Ingår i: Magnetic Resonance Materials in Physics, Biology and Medicine. - 0968-5243.
-
Tidskriftsartikel (refereegranskat)abstract
- Objective: To assess the performance of hybrid multi-dimensional magnetic resonance imaging (HM-MRI) in quantifying hematoxylin and eosin (H&E) staining results, grading and predicting isocitrate dehydrogenase (IDH) mutation status of gliomas. Materials and methods: Included were 71 glioma patients (mean age, 50.17 ± 13.38 years; 35 men). HM-MRI images were collected at five different echo times (80–200 ms) with seven b-values (0–3000 s/mm2). A modified three-compartment model with very-slow, slow and fast diffusion components was applied to calculate HM-MRI metrics, including fractions, diffusion coefficients and T2 values of each component. Pearson correlation analysis was performed between HM-MRI derived fractions and H&E staining derived percentages. HM-MRI metrics were compared between high-grade and low-grade gliomas, and between IDH-wild and IDH-mutant gliomas. Using receiver operational characteristic (ROC) analysis, the diagnostic performance of HM-MRI in grading and genotyping was compared with mono-exponential models. Results: HM-MRI metrics FDvery-slow and FDslow demonstrated a significant correlation with the H&E staining results (p <.05). Besides, FDvery-slow showed the highest area under ROC curve (AUC = 0.854) for grading, while Dslow showed the highest AUC (0.845) for genotyping. Furthermore, a combination of HM-MRI metrics FDvery-slow and T2Dslow improved the diagnostic performance for grading (AUC = 0.876). Discussion: HM-MRI can aid in non-invasive diagnosis of gliomas.
|
|
| 7. |
- Torne, Pablo, et al.
(författare)
-
A Search for Pulsars around Sgr A* in the First Event Horizon Telescope Data Set
- 2023
-
Ingår i: Astrophysical Journal. - 1538-4357 .- 0004-637X. ; 959:1
-
Tidskriftsartikel (refereegranskat)abstract
- In 2017 the Event Horizon Telescope (EHT) observed the supermassive black hole at the center of the Milky Way, Sagittarius A* (Sgr A*), at a frequency of 228.1 GHz (lambda = 1.3 mm). The fundamental physics tests that even a single pulsar orbiting Sgr A* would enable motivate searching for pulsars in EHT data sets. The high observing frequency means that pulsars-which typically exhibit steep emission spectra-are expected to be very faint. However, it also negates pulse scattering, an effect that could hinder pulsar detections in the Galactic center. Additionally, magnetars or a secondary inverse Compton emission could be stronger at millimeter wavelengths than at lower frequencies. We present a search for pulsars close to Sgr A* using the data from the three most sensitive stations in the EHT 2017 campaign: the Atacama Large Millimeter/submillimeter Array, the Large Millimeter Telescope, and the IRAM 30 m Telescope. We apply three detection methods based on Fourier-domain analysis, the fast folding algorithm, and single-pulse searches targeting both pulsars and burst-like transient emission. We use the simultaneity of the observations to confirm potential candidates. No new pulsars or significant bursts were found. Being the first pulsar search ever carried out at such high radio frequencies, we detail our analysis methods and give a detailed estimation of the sensitivity of the search. We conclude that the EHT 2017 observations are only sensitive to a small fraction (less than or similar to 2.2%) of the pulsars that may exist close to Sgr A*, motivating further searches for fainter pulsars in the region.
|
|
| 8. |
- Zhang, Wei, et al.
(författare)
-
Thermal detection of surface plasmons on gold nanohole arrays
- 2012
-
Ingår i: Chinese Science Bulletin. - : Springer Science and Business Media LLC. - 1001-6538 .- 1861-9541. ; 57:1, s. 68-71
-
Tidskriftsartikel (refereegranskat)abstract
- We used a thin-film thermocouple to detect the thermal effect of surface plasmons excited in Au nanohole array structures. We found that the thermal electromotive force of Au film with periodic nanohole structures is three times greater than that of a bare Au film for 785-nm laser excitation at a given power. This effect is caused by the resonant excitation of localized surface plasmons in the nanoholes. In addition, we found that the thermal electromotive force (EMF) of the Au film with dumbbell-like nanohole arrays depends strongly on the incident polarization. The thermal EMF is the greatest when the excitation light is polarized perpendicular to the long axis of the dumbbell.
|
|
| 9. |
- Akiyama, Kazunori, et al.
(författare)
-
First Sagittarius A* Event Horizon Telescope Results. V. Testing Astrophysical Models of the Galactic Center Black Hole
- 2022
-
Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 930:2
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper we provide a first physical interpretation for the Event Horizon Telescope's (EHT) 2017 observations of Sgr A*. Our main approach is to compare resolved EHT data at 230 GHz and unresolved non-EHT observations from radio to X-ray wavelengths to predictions from a library of models based on time-dependent general relativistic magnetohydrodynamics simulations, including aligned, tilted, and stellar-wind-fed simulations; radiative transfer is performed assuming both thermal and nonthermal electron distribution functions. We test the models against 11 constraints drawn from EHT 230 GHz data and observations at 86 GHz, 2.2 mu m, and in the X-ray. All models fail at least one constraint. Light-curve variability provides a particularly severe constraint, failing nearly all strongly magnetized (magnetically arrested disk (MAD)) models and a large fraction of weakly magnetized models. A number of models fail only the variability constraints. We identify a promising cluster of these models, which are MAD and have inclination i <= 30 degrees. They have accretion rate (5.2-9.5) x 10(-9) M (circle dot) yr(-1), bolometric luminosity (6.8-9.2) x 10(35) erg s(-1), and outflow power (1.3-4.8) x 10(38) erg s(-1). We also find that all models with i >= 70 degrees fail at least two constraints, as do all models with equal ion and electron temperature; exploratory, nonthermal model sets tend to have higher 2.2 mu m flux density; and the population of cold electrons is limited by X-ray constraints due to the risk of bremsstrahlung overproduction. Finally, we discuss physical and numerical limitations of the models, highlighting the possible importance of kinetic effects and duration of the simulations.
|
|
| 10. |
- Chen, Zhiwei, et al.
(författare)
-
A Continuous Model for Designing Corridor Systems with Modular Autonomous Vehicles Enabling Station-wise Docking
- 2022
-
Ingår i: Transportation Science. - : Institute for Operations Research and the Management Sciences (INFORMS). - 0041-1655 .- 1526-5447. ; 56:1, s. 1-30
-
Tidskriftsartikel (refereegranskat)abstract
- The "asymmetry" between spatiotemporally varying passenger demand and fixed-capacity transportation supply has been a long-standing problem in urban mass transportation (UMT) systems around the world. The emerging modular autonomous vehicle (MAV) technology offers us an opportunity to close the substantial gap between passenger demand and vehicle capacity through station-wise docking and undocking operations. However, there still lacks an appropriate approach that can solve the operational design problem for UMT corridor systems with MAVs efficiently. To bridge this methodological gap, this paper proposes a continuum approximation (CA) model that can offer near-optimal solutions to the operational design for MAV-based transit corridors very efficiently. We investigate the theoretical properties of the optimal solutions to the investigated problem in a certain (yet not uncommon) case. These theoretical properties allow us to estimate the seat demand of each time neighborhood with the arrival demand curves, which recover the "local impact" property of the investigated problem. With the property, a CA model is properly formulated to decompose the original problem into a finite number of subproblems that can be analytically solved. A discretization heuristic is then proposed to convert the analytical solution from the CA model to feasible solutions to the original problem. With two sets of numerical experiments, we show that the proposed CA model can achieve near-optimal solutions (with gaps less than 4% for most cases) to the investigated problem in almost no time (less than 10 ms) for large-scale instances with a wide range of parameter settings (a commercial solver may even not obtain a feasible solution in several hours). The theoretical properties are verified, and managerial insights regarding how input parameters affect system performance are provided through these numerical results. Additionally, results also reveal that, although the CA model does not incorporate vehicle repositioning decisions, the timetabling decisions obtained by solving the CA model can be easily applied to obtain near-optimal repositioning decisions (with gaps less than 5% in most instances) very efficiently (within 10 ms). Thus, the proposed CA model provides a foundation for developing solution approaches for other problems (e.g., MAV repositioning) with more complex system operation constraints whose exact optimal solution can hardly be found with discrete modeling methods.
|
|
| 11. |
- Cheng, Xiaopeng, et al.
(författare)
-
Toward Microarcsecond Astrometry for the Innermost Wobbling Jet of the BL Lacertae Object OJ 287
- 2023
-
Ingår i: Astrophysical Journal Letters. - 2041-8213 .- 2041-8205. ; 955:2
-
Tidskriftsartikel (refereegranskat)abstract
- The BL Lacertae object OJ 287 is a very unusual quasar producing a wobbling radio jet and some double-peaked optical outbursts with a possible period of about 12 yr for more than one century. This variability is widely explained by models of binary supermassive black holes (SMBHs) or precessing jets/disks from a single SMBH. To enable an independent and nearly bias-free investigation on these possible scenarios, we explored the feasibility of extremely high-precision differential astrometry on its innermost restless jet at millimeter wavelengths. Through revisiting some existing radio surveys and very long baseline interferometry (VLBI) data at frequencies from 1.4 to 15.4 GHz and performing new Very Long Baseline Array observations at 43.2 GHz, we find that the radio source J0854+1959, 7.' 1 apart from OJ 287 and with no clearly seen optical and infrared counterparts, could provide a nearly ideal reference point to track the complicated jet activity of OJ 287. The source J0854+1959 has a stable GHz-peaked radio spectrum and shows a jet structure consisting of two discrete, milliarcsecond-scale-compact and steep-spectrum components and showing no proper motion over about 8 yr. The stable VLBI structure can be interpreted by an episodic, optically thin, and one-sided jet. With respect to its 4.1 mJy peak feature at 43.2 GHz, we have achieved an astrometric precision at the state-of-art level, about 10 mu as. These results indicate that future VLBI astrometry on OJ 287 could allow us to accurately locate its jet apex and activity boundary, align its restless jet structure over decades without significant systematic bias, and probe various astrophysical scenarios.
|
|
| 12. |
- Deng, Min, et al.
(författare)
-
Fine regulation of crystallisation tendency to optimize the BHJ nanostructure and performance of polymer solar cells
- 2020
-
Ingår i: Nanoscale. - : Royal Society of Chemistry. - 2040-3364 .- 2040-3372. ; 12:24, s. 12928-12941
-
Tidskriftsartikel (refereegranskat)abstract
- Optimizing the nanostructure of the active layer of polymer solar cells (PSCs) is one of the main challenges to achieve high device performances. The phase separation of the donor polymer and molecular acceptor within the bulk heterojunction (BHJ) layer is often driven by the crystallisation of the acceptor molecules. Hence, a suitable crystallisation tendency of the chosen acceptor is ultimately important. In this work, we identified melting temperature as an indicator for the crystallisation tendency and introduced extended fused-aromatic rings to the end groups of the nonfullerene acceptor molecule to enhance the intermolecular binding energy as well as its crystallisation tendency. The crystallinity, crystal regularity and average crystal size were significantly increased for those molecules with larger fused end groups. The devices containing molecule IDTTC with two fused thiophene rings, which displayed intermediate crystallisation tendency, were found to possess an optimized phase separation scale, balanced hole/electron mobility and highest device performances with the fill factor as high as 73.2% and a power conversion efficiency of 13.49%. With the above observations, we established a new route and paradigm to adjust the crystallisation tendency and BHJ nanostructure of nonfullerene acceptor molecules, thus enhancing the device performances through molecular engineering.
|
|
| 13. |
- Foox, Jonathan, et al.
(författare)
-
The SEQC2 epigenomics quality control (EpiQC) study
- 2021
-
Ingår i: Genome Biology. - : BioMed Central (BMC). - 1465-6906 .- 1474-760X. ; 22:1
-
Tidskriftsartikel (refereegranskat)abstract
- BackgroundCytosine modifications in DNA such as 5-methylcytosine (5mC) underlie a broad range of developmental processes, maintain cellular lineage specification, and can define or stratify types of cancer and other diseases. However, the wide variety of approaches available to interrogate these modifications has created a need for harmonized materials, methods, and rigorous benchmarking to improve genome-wide methylome sequencing applications in clinical and basic research. Here, we present a multi-platform assessment and cross-validated resource for epigenetics research from the FDA’s Epigenomics Quality Control Group.ResultsEach sample is processed in multiple replicates by three whole-genome bisulfite sequencing (WGBS) protocols (TruSeq DNA methylation, Accel-NGS MethylSeq, and SPLAT), oxidative bisulfite sequencing (TrueMethyl), enzymatic deamination method (EMSeq), targeted methylation sequencing (Illumina Methyl Capture EPIC), single-molecule long-read nanopore sequencing from Oxford Nanopore Technologies, and 850k Illumina methylation arrays. After rigorous quality assessment and comparison to Illumina EPIC methylation microarrays and testing on a range of algorithms (Bismark, BitmapperBS, bwa-meth, and BitMapperBS), we find overall high concordance between assays, but also differences in efficiency of read mapping, CpG capture, coverage, and platform performance, and variable performance across 26 microarray normalization algorithms.ConclusionsThe data provided herein can guide the use of these DNA reference materials in epigenomics research, as well as provide best practices for experimental design in future studies. By leveraging seven human cell lines that are designated as publicly available reference materials, these data can be used as a baseline to advance epigenomics research.
|
|
| 14. |
- Fu, Hao, et al.
(författare)
-
A compact real-time simulator with spatial-temporal parallel design for large-scale wind farms
- 2021
-
Ingår i: CSEE Journal of Power and Energy Systems. - : Power System Technology Press. - 2096-0042. ; 9:1, s. 50-65
-
Tidskriftsartikel (refereegranskat)abstract
- Real-time simulation of large-scale wind farms with detailed modelling can provide accurate insights into system transient behaviors, but entails challenges in computing resources. This paper develops a compact real-time simulator based on field programmable gate array (FPGA) for large-scale wind farms, in which the spatial-temporal parallel design method is proposed to address the huge computation resource demand associated with detailed modelling. The wind farm is decoupled into several subsystems based on model consistency, and the electrical system and control system of each subsystem are solved in parallel. Both module-level pipeline technique and superscalar pipeline technique are introduced to the wind farms' simulation to effectively improve the utilization of hardware resources. In Case Studies, real-time simulations of two modified wind farms are carried out on a single FPGA separately, including one with 13 permanent magnet synchronous generators under a time-step of 11 μβ, and the other with 30 squirrel-cage induction generators under a time-step of 8μβ. Simulation tests under different scenarios are implemented to validate the numerical performance of the real-time simulator, and a comparison with the commercial tool PSCAD/EMTDC demonstrates the accuracy and effectiveness of the proposed design.
|
|
| 15. |
- Fu, Hao, et al.
(författare)
-
Asynchronous multi-rate method of real-time simulation for active distribution networks
- 2023
-
Ingår i: IET Renewable Power Generation. - : INST ENGINEERING TECHNOLOGY-IET. - 1752-1416 .- 1752-1424.
-
Tidskriftsartikel (refereegranskat)abstract
- The real-time simulation of active distribution networks (ADNs) can provide an accurate insight into transient behaviours, but faces challenges in simulation efficiency and flexibility brought by larger system scales and wider time-scale ranges. This paper presents an asynchronous multi-rate (AMR) method and design for the real-time simulation of large-scale ADNs. In the proposed method, the entire ADN was decoupled into different subsystems according to accuracy requirements, and optimized time-steps were allocated to each subsystem to realize a fully distributed simulation. This not only alleviated the time-step coordination problem existing in multi-rate real-time simulations, but also enhanced the flexible expansion capabilities of the real-time simulator. To realize the AMR real-time simulation, a multi-rate interfacing method, synchronization mechanism, and data communication strategy are proposed in this paper, and their hardware design is also presented in detail. A modified IEEE 123-node system with photovoltaics and wind turbine generators was simulated on a 3 field-programmable gate arrays (FPGAs)-based AMR real-time simulator. The real-time results were captured by the oscilloscope and verified with PSCAD/EMTDC, which demonstrated the superiority in simulation flexibility and accuracy compared with the synchronous multi-rate (SMR) method.
|
|
| 16. |
- Georgiev, Boris, et al.
(författare)
-
A Universal Power-law Prescription for Variability from Synthetic Images of Black Hole Accretion Flows
- 2022
-
Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 930:2
-
Tidskriftsartikel (refereegranskat)abstract
- We present a framework for characterizing the spatiotemporal power spectrum of the variability expected from the horizon-scale emission structure around supermassive black holes, and we apply this framework to a library of general relativistic magnetohydrodynamic (GRMHD) simulations and associated general relativistic ray-traced images relevant for Event Horizon Telescope (EHT) observations of Sgr A*. We find that the variability power spectrum is generically a red-noise process in both the temporal and spatial dimensions, with the peak in power occurring on the longest timescales and largest spatial scales. When both the time-averaged source structure and the spatially integrated light-curve variability are removed, the residual power spectrum exhibits a universal broken power-law behavior. On small spatial frequencies, the residual power spectrum rises as the square of the spatial frequency and is proportional to the variance in the centroid of emission. Beyond some peak in variability power, the residual power spectrum falls as that of the time-averaged source structure, which is similar across simulations; this behavior can be naturally explained if the variability arises from a multiplicative random field that has a steeper high-frequency power-law index than that of the time-averaged source structure. We briefly explore the ability of power spectral variability studies to constrain physical parameters relevant for the GRMHD simulations, which can be scaled to provide predictions for black holes in a range of systems in the optically thin regime. We present specific expectations for the behavior of the M87* and Sgr A* accretion flows as observed by the EHT.
|
|
| 17. |
- Guo, Lijie, et al.
(författare)
-
Industrial practice on optimizing tailings composition combined with ore concentration processes
- 2017
-
Ingår i: 12th International Conference on Mining with Backfill 2017, Minefill 2017. - Englewood, CO : Society for Mining, Metallurgy and Exploration. - 9781510835818 ; , s. 69-79
-
Konferensbidrag (refereegranskat)abstract
- Cemented paste backfill (CPB) is increasingly applied in underground mines world-wide and has a broad prospect considering its benefits in environmental protection and ground control. During the paste production, a well-graded aggregate is critical for the achievement of CPB. Mine tailings from the outcome of ore-dressing plant are commonly used as the fill aggregate. However, it is difficult to obtain an ideal particle size distribution for the mine tailings which are roughly mixed with final residuum in ore concentration processes. In this study, a novel industrial method is proposed to select fill aggregates from different parts of ore-dressing processes. With combined design of concentration plant and backfill plant, the tailings with desired particle sizes and proportions during the mineral processing are chosen to produce CPB without disturbing the regular ore concentration. The problem of passively accepting final tailings from ore-dressing plant has been improved
|
|
| 18. |
- Jiang, Yonglei, et al.
(författare)
-
Rotation cropping and organic fertilizer jointly promote soil health and crop production
- 2022
-
Ingår i: Journal of Environmental Management. - : Elsevier BV. - 0301-4797. ; 315
-
Tidskriftsartikel (refereegranskat)abstract
- Identifying field management practices to promote crop production, while conserving soil health is essential to maintain long-term food production in a changing world. Also, providing experimental evidence to support the use of traditional agricultural practices is necessary to secure sustainable agriculture. Here, we conducted a long-term 12-year experiment to investigate the impact of different combinations of fertilization type (control, inorganic fertilizer, organic fertilizer) and cropping regimes (continuous cropping and rotation cropping) on the crop (tobacco) production and multiple soil attributes associated with soil health, including proportions of soil-borne pathogens and decomposers, soil microbial diversity, microbial network stability and biomass, nutrient pools and microbial resource limitations. Our long-term experiment supports that the combination of organic fertilizer with rotation cropping increased crop production by at least 40% compared to the other management combinations and improved soil nutrient pools (e.g. the content of soil organic matter), improved the relative proportion of soil decomposers, and promoted bacterial and fungal network stability and biodiversity. Furthermore, this combination treatment relieved microbial resource limitation and reduced the abundance of potential fungal plant pathogens by at least 20% compared to other management combinations. In summary, we provide experimental evidence to support that the combined use of organic fertilization and rotation cropping management can help maintain long-term soil health, crop production, and economic outputs.
|
|
| 19. |
- Jorstad, S.G., et al.
(författare)
-
The Event Horizon Telescope Image of the Quasar NRAO 530
- 2023
-
Ingår i: Astrophysical Journal. - : American Astronomical Society. - 1538-4357 .- 0004-637X. ; 943:2
-
Tidskriftsartikel (refereegranskat)abstract
- We report on the observations of the quasar NRAO 530 with the Event Horizon Telescope (EHT) on 2017 April 5-7, when NRAO 530 was used as a calibrator for the EHT observations of Sagittarius A*. At z = 0.902, this is the most distant object imaged by the EHT so far. We reconstruct the first images of the source at 230 GHz, at an unprecedented angular resolution of similar to 20 mu as, both in total intensity and in linear polarization (LP). We do not detect source variability, allowing us to represent the whole data set with static images. The images reveal a bright feature located on the southern end of the jet, which we associate with the core. The feature is linearly polarized, with a fractional polarization of similar to 5%-8%, and it has a substructure consisting of two components. Their observed brightness temperature suggests that the energy density of the jet is dominated by the magnetic field. The jet extends over 60 mu as along a position angle similar to -28 degrees. It includes two features with orthogonal directions of polarization (electric vector position angle), parallel and perpendicular to the jet axis, consistent with a helical structure of the magnetic field in the jet. The outermost feature has a particularly high degree of LP, suggestive of a nearly uniform magnetic field. Future EHT observations will probe the variability of the jet structure on microarcsecond scales, while simultaneous multiwavelength monitoring will provide insight into the high-energy emission origin.
|
|
| 20. |
- Li, Shen, et al.
(författare)
-
First-principles investigation of electronic structure and transport properties of CoSb3 under different pressures
- 2012
-
Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 549, s. 22-26
-
Tidskriftsartikel (refereegranskat)abstract
- The electronic structure of pure CoSb3 (undoped) and the electronic transport properties as a function of chemical potential (mu) of CoSb3 under different pressures were studied systematically and in detailin our research. Direct band gap change to indirect band gap with increasing pressure was found for the undoped CoSb3. The electrical conductivity of undoped CoSb3 significantly decreases with increasing pressure from 0 to 10 GPa, the Seebeck coefficient of undoped CoSb3 significantly enhanced with increasing pressure. The n-type of doping in CoSb3 would be more favorable for enhancing the thermoelectric properties than that of p-type doping under pressure.
|
|
| 21. |
|
|
| 22. |
- Li, Xiaopeng
(författare)
-
Effects of ageing on contractility and myosin composition of rat and man skeletal muscles and muscle cells : with special reference to muscle adaptability
- 1996
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aims of this thesis were to characterize the effects of ageing on the contractile and biochemical properties of fast- and slow-twitch skeletal muscles at the whole muscle and single fibre level in an attempt to improve our understanding of the mechanisms underlying the age-related impairments of muscle fibres. Specific interest was in the adaptability of skeletal muscle and the regulation of speed of shortening at single-cell level in the rat and man. At the whole-muscle level in the rat, an age-related increase in contraction and half-relaxation times of the isometric twitch was observed in both fast- and slow-twitch muscles, as well as an age-related increase in the proportion of type I, and decrease in type IIA, fibres in the slow-twitch soleus. Four weeks of T3 treatment eliminated the age-related differences in isometric twitch properties in both soleus and EDL muscles. T3 induced a dramatic decrease in the proportion of type I fibres from young and old soleus, with a concomitant increase in the relative number of type IC and IIC fibres. But the fibre type composition of EDL was not affected either by ageing or by T3 treatment. In the rat soleus, the type I MyHC content was higher and type IIA content lower in old rats than in the young. Type IIXMyHC was observed in some young control soleus muscles, but not in old ones. After four weeks of T3 treatment, the content of type I MyHC decreased and type IIA and IIX increased in both young and old animals, resulting in an elimination of the age-related difference in the contents of type I and 11 MyHCs. In the EDL, the content of type IIX MyHC was significantly higher in the old animals, at the expense of a lower content of type IIB MyHC. MyHC composition was not affected significantly by T, treatment in EDL, irrespective of age. In single skinned fibre preparations, V0 was significantly lower in soleus fibres from old control rats than from young ones, despite the fact that all fibres expressed the type I MyHC and slow MyLC isoforms. In the EDL, V0 values in single fibres did not differ between young and old animals. Further, the coordinated expression of MyLC and MyHC isoforms in type IIXB fibres, as well as the correlation between V0 and the compositions of MyLC and MyHC, that was observed in young rats was lost in old animals. We speculate that there could be more than one beta/slow MyHC isoform and an age-related transition within these isoforms. Thyroid hormone induced a dramatic up-regulation of type IIA or/and IIX MyHCs as well as the fast isoforms of MyLCs in soleus muscle fibres, but variable amounts of type I MyHC were still observed in almost all single soleus cells in both young and old animals. T3 had no detectable effect on V0 in young soleus fibres, but significantly increased VO in old soleus fibres. However, the age-related decline in V0 still persisted in the soleus fibres. Myosin composition and VO of EDL fibres were not affected by T3 treatment, either in young or old rats. There could be slow MyHC diversity not detected by our methods or the MyHC ATPase rate may be regulated by mechanisms other than isoform transitions as fibres become older, and this effect is partially reversed by T3. In man, muscle biopsies were obtained from young, sedentary old and well trained old male volunteers. Type I MyHC fibres were more frequent in the old subjects than in the young and well trained old ones. Myofibrillar ATPase (mATPase) activities were lower in all fibre types in the old as compared to the young subjects, while mATPase in fibres from well-trained old men were intermediate between those of young and old controls. MyLC3 was observed in many type I MyHC fibres, irrespective of age. The type I fibres expressing both slow and fast isoforms of MyLC2 were more frequent in the young subjects than in the old. In parallel with the age-related decrease in mATPase activity, age-related declines in V0 and specific tension were observed in type I and IIA MyHC fibres according to 6% SDS-PAGE. Physical activity in the old men counteracted the age-related atrophy of fast muscle cells, but V0 as well as specific tension was still lower in the physically active old subjects than in young controls. In conclusion, the altered contractile properties in type I and IIA MyHC fibres are in pan related to changes in enzymatic properties of myosin and in the composition of MyLC isoforms. However, other etiological factors also have to be considered such as an incomplete characterization of MyHCs or the fact that other thick and thin filament proteins influencing contractility are altered in old age.
|
|
| 23. |
- Li, Xiaopeng, et al.
(författare)
-
Playful Advertising : In-Game Advertising for Virtual Reality Games
- 2015
-
Ingår i: SIGRAD 2015. - Linköping. ; , s. 46-48
-
Konferensbidrag (refereegranskat)abstract
- We present an early exploration of in-game advertising for virtual reality games. First, we establish a theoretical grounding for understanding interactivity and immersion in virtual reality games. Next, we report the results of a number of field studies, expert interviews, prototype designs, and describe the design of a pilot user study. Based on these results, we discuss the design of interactivity and immersion for in-game advertising and the impacts on consumer learning and game experience.
|
|
| 24. |
- Li, Xiaopeng (Shaw), et al.
(författare)
-
Emerging Mobility Systems
- 2019
-
Ingår i: IEEE Intelligent Transportation Systems Magazine. - 1939-1390 .- 1941-1197. ; 11:3, s. 8-11
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
|
|
| 25. |
- Qu, Xiaobo, 1983, et al.
(författare)
-
Contagion spread modeling in transport networks and transport operation optimizations for containing epidemics
- 2022
-
Ingår i: Transportation Amid Pandemics: Lessons Learned from COVID-19. ; , s. 349-357
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- COVID-19 has critically impacted many aspects of societies worldwide, particularly on mobility. This chapter summarizes impacts of the COVID-19 pandemic, reviews existing research, and identifies future research needs in the scope of traffic theory and modeling/optimization and traffic flow. We first review models on contagion spreading through transportation networks, including aggregated spatial metapopulation models and disaggregated individual-based models. Further research is needed to consider both intercity and intracity mobilities and leverage emerging multiple data resources for constructing individuals’ complete trip chains. Based on modeling contagion spreading, we further discuss transport operation needs in the aftermath of COVID-19. There remains a need for operating multimodal urban transport systems to satisfy basic travel demands while minimizing contagion risks. Relevant research needs are identified in optimizing transport operation via modern data acquisition technologies and advanced modeling methods. Practical intervention measures and policy implications are recommended for optimizing transport systems during the COVID-19 pandemic.
|
|
| 26. |
- Shi, Xiaowei, et al.
(författare)
-
The effect of ride experience on changing opinions toward autonomous vehicle safety
- 2021
-
Ingår i: Communications in Transportation Research. - : Elsevier BV. - 2772-4247. ; 1
-
Tidskriftsartikel (refereegranskat)abstract
- Autonomous vehicles (AVs) are a promising emerging technology that is likely to be widely deployed in the near future. People's perception on AV safety is critical to the pace and success of deploying the AV technology. Existing studies found that people's perceptions on emerging technologies might change as additional information was provided. To investigate this phenomenon in the AV technology context, this paper conducted real-world AV experiments and collected factors that may associate with people's initial opinions without any AV riding experience and opinion change after a successful AV ride. A number of ordered probit and binary probit models considering data heterogeneity were employed to estimate the impact of these factors on people's initial opinions and opinion change. The study found that people's initial opinions toward AV safety are significantly associated with people's age, personal income, monthly fuel cost, education experience, and previous AV experience. Further, the factors dominating people's opinion change after a successful AV ride include people's age, personal income, monthly fuel cost, daily commute time, driving alone indicator, willingness to pay for AV technology, and previous AV experience. These results provide important references for future implementations of the AV technology. Additionally, based on the inconsistent effects for variables across different models, suggestions for future transportation survey designs are provided.
|
|
| 27. |
|
|
| 28. |
- Vanmeensel, Kim, et al.
(författare)
-
8 - Additively manufactured metals for medical applications
- 2018
-
Ingår i: Additive Manufacturing. - : Butterworth-Heinemann. - 9780128121559 ; , s. 261-309
-
Bokkapitel (refereegranskat)abstract
- Additive Manufacturing (AM) allows to produce improved, custom made, patient specific and complex shaped medical implants. Both mechanical and biological aspects as well as type of implant and patient specific requirements determine the shape of the implant and the material it will be made of. The current chapter reviews the metals and metallic alloys that can be successfully processed by powder bed fusion AM technologies, focusing on the interrelationship between their chemistry and microstructure, on one hand, and their biological and mechanical behavior, respectively, on the other hand.
|
|
| 29. |
- Wang, Liang, et al.
(författare)
-
Longitudinal control for person-following robots
- 2022
-
Ingår i: Journal of Intelligent and Connected Vehicles. - 2399-9802. ; 5:2, s. 88-98
-
Tidskriftsartikel (refereegranskat)abstract
- Purpose: This paper aims to address the longitudinal control problem for person-following robots (PFRs) for the implementation of this technology. Design/methodology/approach: Nine representative car-following models are analyzed from PFRs application and the linear model and optimal velocity model/full velocity difference model are qualified and selected in the PFR control. Findings: A lab PFR with the bar-laser-perception device is developed and tested in the field, and the results indicate that the proposed models perform well in normal person-following scenarios. Originality/value: This study fills a gap in the research on PRFs longitudinal control and provides a useful and practical reference on PFRs longitudinal control for the related research.
|
|
| 30. |
- Wang, Yan Ming, et al.
(författare)
-
Colorado geoid computation experiment : overview and summary
- 2021
-
Ingår i: Journal of Geodesy. - : Springer. - 0949-7714 .- 1432-1394. ; 95:12
-
Tidskriftsartikel (refereegranskat)abstract
- The primary objective of the 1-cm geoid experiment in Colorado (USA) is to compare the numerous geoid computation methods used by different groups around the world. This is intended to lay the foundations for tuning computation methods to achieve the sought after 1-cm accuracy, and also evaluate how this accuracy may be robustly assessed. In this experiment, (quasi)geoid models were computed using the same input data provided by the US National Geodetic Survey (NGS), but using different methodologies. The rugged mountainous study area (730 km x 560 km) in Colorado was chosen so as to accentuate any differences between the methodologies, and to take advantage of newly collected GPS/leveling data of the Geoid Slope Validation Survey 2017 (GSVS17) which are now available to be used as an accurate and independent test dataset. Fourteen groups from fourteen countries submitted a gravimetric geoid and a quasigeoid model in a 1' x 1' grid for the study area, as well as geoid heights, height anomalies, and geopotential values at the 223 GSVS17 marks. This paper concentrates on the quasigeoid model comparison and evaluation, while the geopotential value investigations are presented as a separate paper (Sanchez et al. in J Geodesy 95(3):1. https://doi.org/10.1007/s00190-021-01481-0, 2021). Three comparisons are performed: the area comparison to show the model precision, the comparison with the GSVS17 data to estimate the relative accuracy of the models, and the differential quasigeoid (slope) comparison with GSVS17 to assess the relative accuracy of the height anomalies at different baseline lengths. The results show that the precision of the 1' x 1' models over the complete area is about 2 cm, while the accuracy estimates along the GSVS17 profile range from 1.2 cm to 3.4 cm. Considering that the GSVS17 does not pass the roughest terrain, we estimate that the quasigeoid can be computed with an accuracy of similar to 2 cm in Colorado. The slope comparisons show that RMS values of the differences vary from 2 to 8 cm in all baseline lengths. Although the 2-cm precision and 2-cm relative accuracy have been estimated in such a rugged region, the experiment has not reached the 1-cm accuracy goal. At this point, the different accuracy estimates are not a proof of the superiority of one methodology over another because the model precision and accuracy of the GSVS17-derived height anomalies are at a similar level. It appears that the differences are not primarily caused by differences in theory, but that they originate mostly from numerical computations and/or data processing techniques. Consequently, recommendations to improve the model precision toward the 1-cm accuracy are also given in this paper.
|
|
| 31. |
- Wang, Zhen, et al.
(författare)
-
Modeling and field experiments on autonomous vehicle lane changing with surrounding human-driven vehicles
- 2021
-
Ingår i: Computer-Aided Civil and Infrastructure Engineering. - : Wiley. - 1093-9687 .- 1467-8667. ; 36:7, s. 877-889
-
Tidskriftsartikel (refereegranskat)abstract
- Autonomous vehicle (AV) technology is widely studied in both industrial and academic communities since it is regarded as a promising means for improving transportation safety and efficiency. Lane changing is a critical link for higher-level AV operations. However, few studies on AV lane changing consider the dynamics of surrounding vehicles, particularly in a mixed traffic environment including human-driven vehicles (HVs). Therefore, this article presents a dynamic lane-changing model for AV incorporating human driver behavior in mixed traffic. The proposed model includes four key components: car following (and lane keeping), lane-changing decision, dynamic trajectory generation, and model predictive control (MPC)-based trajectory tracking. AV longitudinal control algorithm is also depicted in detail in this article. Field experiments are conducted on a large-scale test track to test and validate the proposed model. An AV and three HVs are used in the lane-changing experiments. Different human driver behaviors are considered in the experiment settings. Experimental results show that the proposed lane-changing model can complete lane-changing maneuvers efficiently when HVs are cooperative and can also robustly abort them when HVs are uncooperative. Compared with the measured human lane-changing maneuvers, AV lane-changing maneuvers from the proposed model are more comfortable and safer.
|
|
| 32. |
- Xu, Zhigang, et al.
(författare)
-
Trajectory Optimization for a Connected Automated Traffic Stream: Comparison Between an Exact Model and Fast Heuristics
- 2021
-
Ingår i: IEEE Transactions on Intelligent Transportation Systems. - 1524-9050 .- 1558-0016. ; 22:5, s. 2969-2978
-
Tidskriftsartikel (refereegranskat)abstract
- Numerous fast heuristic algorithms, including shooting heuristics (SH), have been developed for real-time trajectory optimization, although their optimality has not yet been quantified. This paper compares the performance between fast heuristics and exact optimization models. We investigate a core trajectory optimization problem as a building block for numerous trajectory optimization problems, i.e., guiding movements of connected automated vehicles on a one-lane highway when the arrival and departure times and velocity are given. To apply the SH algorithm to this problem, we adapt it to a fast-simplified shooting heuristic (FSSH) model to solve the trajectory smoothing problems with different arrival and departure velocities. An exact trajectory optimization (ETO) model is formulated that takes the vehicle position and velocity as the decision variables, and the fuel consumption and driving comfort as the objective function. The constraints of the model are based on the limits and safety of the vehicle dynamics between consecutive vehicles. We demonstrate the convexity of the ETO objective function, ensuring the solvability of the ETO model at the true optimum using gradient descent algorithms supplied by the MATLAB optimization toolbox. Six groups of numerical experiments using different input parameters and one experiment using real Next Generation Simulation (NGSIM) data are conducted. ETO can improve the objective values by a few to tens of percentage points. However, FSSH achieves a greater solution efficiency with an average solution time of less than 0.1 s compared to similar to 450 s for ETO.
|
|
| 33. |
- Zhou, Mofan, et al.
(författare)
-
On the impact of cooperative autonomous vehicles in improving freeway merging: A modified intelligent driver model based approach
- 2017
-
Ingår i: IEEE Transactions on Intelligent Transportation Systems. - 1524-9050 .- 1558-0016. ; 18:6, s. 1422-1428
-
Tidskriftsartikel (refereegranskat)abstract
- Transport researchers and practitioners have long been seeking capable solutions to deal with the traffic oscillations caused by freeway merging. Although existing approaches based on ramp metering have improved the overall efficiency of on-ramps, their performance is still far below the theoretical capacity. The recently proposed detecting technology of autonomous vehicles (AVs) provides an alternative for maximizing the merging efficiency by developing and using appropriate controllers for AVs. In this paper, we develop a cooperative intelligent driver model in order to examine the system performance under different proportions of AVs. The results show that, with a proper vehicle-to-vehicle controlling mechanism, an increasing percentage of AVs will reduce the total travel time and smooth traffic oscillations.
|
|
