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Träfflista för sökning "WFRF:(Witzel A.) "

Sökning: WFRF:(Witzel A.)

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1.
  • Marcaide, J. M., et al. (författare)
  • Discovery of shell-like radio-structure in SN1993J
  • 1995
  • Ingår i: Nature. - London : Nature Publishing Group. - 0028-0836 .- 1476-4687. ; 373:6509, s. 44-45
  • Tidskriftsartikel (refereegranskat)abstract
    • SUPERNOVA explosions are poorly understood, partly because of difficulties in modelling them theoretically(1), and partly because there have been no supernovae observed in our Galaxy since the invention of the telescope. But the recent discovery(2) of supernova SN1993J in the nearby galaxy M81 offers an opportunity to investigate the evolution of the remnant, and its interaction with the surrounding interstellar medium, at high resolution. Here we present radio observations of SN1993J, made using very-long-baseline interferometry, which show the development of a shell structure. This 8-month-old radio shell is the youngest ever discovered in a supernova. The data suggest that the supernova explosion and the expanding shell of the remnant have nearly spherical symmetry, with small deviations where some parts of the shell are brighter than others. If these deviations arise because of variations in the density of the shell, this may reconcile earlier reports of symmetric radio emission(3) with the observed optical asymmetry(4,5), as the density variations could easily cause the latter. We infer that the radio emission is generated at the interface(6-9), where the surrounding gas is shocked by the ejecta.
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2.
  • Pagels, A., et al. (författare)
  • Millimetre VLBI monitoring of AGN with sub-milliarcsecond resolution
  • 2004
  • Ingår i: in "Proceedings of the 7th European VLBI Network Symposium", Oct. 2004, Toledo, eds. R. Bachiller, F. Colomer, J.F. Desmurs, P. de Vicente, Obs. Astronómico Nacional, Alcala de Henares, Spain. ; , s. 7-10
  • Konferensbidrag (övrigt vetenskapligt)
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3.
  • Taquet, V, et al. (författare)
  • Seeds of Life in Space (SOLIS) VI. Chemical evolution of sulfuretted species along the outflows driven by the low-mass protostellar binary NGC1333-IRAS4A
  • 2020
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 637
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Low-mass protostars drive powerful molecular outflows that can be observed with millimetre and submillimetre telescopes. Various sulfuretted species are known to be bright in shocks and could be used to infer the physical and chemical conditions throughout the observed outflows. Aims. The evolution of sulfur chemistry is studied along the outflows driven by the NGC1333-IRAS4A protobinary system located in the Perseus cloud to constrain the physical and chemical processes at work in shocks. Methods. We observed various transitions from OCS, CS, SO, and SO2 towards NGC1333-IRAS4A in the 1.3, 2, and 3mm bands using the IRAM NOrthern Extended Millimeter Array and we interpreted the observations through the use of the Paris-Durham shock model. Results. The targeted species clearly show different spatial emission along the two outflows driven by IRAS4A. OCS is brighter on small and large scales along the south outflow driven by IRAS4A1, whereas SO2 is detected rather along the outflow driven by IRAS4A2 that is extended along the north east-south west direction. SO is detected at extremely high radial velocity up to +25 km s 1 relative to the source velocity, clearly allowing us to distinguish the two outflows on small scales. Column density ratio maps estimated from a rotational diagram analysis allowed us to confirm a clear gradient of the OCS/SO2 column density ratio between the IRAS4A1 and IRAS4A2 outflows. Analysis assuming non Local Thermodynamic Equilibrium of four SO2 transitions towards several SiO emission peaks suggests that the observed gas should be associated with densities higher than 105 cm 3 and relatively warm (T > 100 K) temperatures in most cases. Conclusions. The observed chemical differentiation between the two outflows of the IRAS4A system could be explained by a different chemical history. The outflow driven by IRAS4A1 is likely younger and more enriched in species initially formed in interstellar ices, such as OCS, and recently sputtered into the shock gas. In contrast, the longer and likely older outflow triggered by IRAS4A2 is more enriched in species that have a gas phase origin, such as SO2.
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4.
  • Jones, D. L., et al. (författare)
  • High dynamic range VLBI observations of NGC 6251
  • 1986
  • Ingår i: Astrophysical Journal. - Philadelphia, United States : Institute of Physics Publishing, Inc. - 0004-637X .- 1538-4357. ; 305:2, s. 684-697
  • Tidskriftsartikel (refereegranskat)
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5.
  • Ferizi, Uran, et al. (författare)
  • Diffusion MRI microstructure models with in vivo human brain Connectome data : Results from a multi-group comparison
  • 2017
  • Ingår i: NMR in Biomedicine. - : John Wiley and Sons. - 0952-3480 .- 1099-1492. ; 30:9
  • Tidskriftsartikel (refereegranskat)abstract
    • A large number of mathematical models have been proposed to describe the measured signal in diffusion-weighted (DW) magnetic resonance imaging (MRI). However, model comparison to date focuses only on specific subclasses, e.g. compartment models or signal models, and little or no information is available in the literature on how performance varies among the different types of models. To address this deficiency, we organized the 'White Matter Modeling Challenge' during the International Symposium on Biomedical Imaging (ISBI) 2015 conference. This competition aimed to compare a range of different kinds of models in their ability to explain a large range of measurable in vivo DW human brain data. Specifically, we assessed the ability of models to predict the DW signal accurately for new diffusion gradients and b values. We did not evaluate the accuracy of estimated model parameters, as a ground truth is hard to obtain. We used the Connectome scanner at the Massachusetts General Hospital, using gradient strengths of up to 300mT/m and a broad set of diffusion times. We focused on assessing the DW signal prediction in two regions: the genu in the corpus callosum, where the fibres are relatively straight and parallel, and the fornix, where the configuration of fibres is more complex. The challenge participants had access to three-quarters of the dataset and their models were ranked on their ability to predict the remaining unseen quarter of the data. The challenge provided a unique opportunity for a quantitative comparison of diverse methods from multiple groups worldwide. The comparison of the challenge entries reveals interesting trends that could potentially influence the next generation of diffusion-based quantitative MRI techniques. The first is that signal models do not necessarily outperform tissue models; in fact, of those tested, tissue models rank highest on average. The second is that assuming a non-Gaussian (rather than purely Gaussian) noise model provides little improvement in prediction of unseen data, although it is possible that this may still have a beneficial effect on estimated parameter values. The third is that preprocessing the training data, here by omitting signal outliers, and using signal-predicting strategies, such as bootstrapping or cross-validation, could benefit the model fitting. The analysis in this study provides a benchmark for other models and the data remain available to build up a more complete comparison in the future.
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6.
  • Rantakyrö, F.T., et al. (författare)
  • 50 μas resolution VLBI images of AGN’s at λ3 mm
  • 1998
  • Ingår i: Astronomy and Astrophysics. - Berlin : Springer-Verlag. - 0004-6361 .- 1432-0746. ; 131, s. 451-467
  • Tidskriftsartikel (refereegranskat)abstract
    • We present 15 images from the global mm-VLBI sessions in 1990 April at 100 GHz and 1993 April at 86 GHz. These observations probe the central engines of the 16 observed AGN's with up to 50 mu as resolution. Among other sources previously observed with lambda 3 mm VLBI we present the first lambda 3 mm maps of 0735+178, 0748+126, 1055+018, 2145+067, and CTA102, in total we have been able to image 13 out of the 16 observed sources. 6 out of the 13 imaged sources observed exhibit curvature and rapid structural changes, although the low dynamic range in two thirds of the maps limits the detection of weak features. Most of the sources have unresolved cores even at this high resolution. There is substantial evidence that the observed sources can be grouped into two general groups: A misaligned population with parsec scale jets in the form of low pitch helices and an aligned population with straight jets with small changes in PA due to intrinsic bends.
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7.
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8.
  • Standke, K. J., et al. (författare)
  • High Sensitivity VLBI at 86 GHz : First Fringes with the 100 m Radio Telescope at Effelsberg
  • 1993
  • Ingår i: VLBI Technology. - Tokyo : Terra Scientific. - 4887041128 ; , s. 75-85
  • Bokkapitel (refereegranskat)abstract
    • Inclusion of large and sensitive antennas is of essential importance to further improve the imaging capabilities of VLBI at 3.5 mm wavelength. The combination of the 100 m radio telescope at Effelsberg (illuminated diameter ~60 m at lambda = 3.5 mm) with the 30 m radio telescope at Pico Veleta presently allows the detection of compact flux down to a level of 0.2 Jy (for τ=360 seconds integration time). Fringes have been found for 23 radio sources many of them not observed previously with VLBI at 86 GHz. On European baselines to Effelsberg the SNR typically was 10 to 30 (and ranged up to 107 for 3C279), facilitating model fitting for the brightest sources. Even in spite of chaning weather conditions during the observations (coherence times ranged between 10 to 100 seconds) and a still non-optimum pointing model for the 100 m antenna for observations at high frequencies, these results demonstrate that the 100 m radio telescope can be successfully incorporated in global VLBI experiments at 3.5 mm wavelength.
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9.
  • Alniss, Hasan Y., et al. (författare)
  • Investigation of the Factors That Dictate the Preferred Orientation of Lexitropsins in the Minor Groove of DNA
  • 2019
  • Ingår i: Journal of Medicinal Chemistry. - : AMER CHEMICAL SOC. - 0022-2623 .- 1520-4804. ; 62:22, s. 10423-10440
  • Tidskriftsartikel (refereegranskat)abstract
    • Lexitropsins are small molecules that bind to the minor groove of DNA as antiparallel dimers in a specific orientation. These molecules have shown therapeutic potential in the treatment of several diseases; however, the development of these molecules to target particular genes requires revealing the factors that dictate their preferred orientation in the minor grooves, which to date have not been investigated. In this study, a distinct structure (thzC) was carefully designed as an analog of a well-characterized lexitropsin (thzA) to reveal the factors that dictate the preferred binding orientation. Comparative evaluations of the biophysical and molecular modeling results of both compounds showed that the position of the dimethylaminopropyl group and the orientation of the amide links of the ligand with respect to the 5'-3'-ends; dictate the preferred orientation of lexitropsins in the minor grooves. These findings could be useful in the design of novel lexitropsins to selectively target specific genes.
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10.
  • Bååth, L.B. 1948-, et al. (författare)
  • High Resolution Observations of the QSO 3C 345 at 1.3 centimeters
  • 1981
  • Ingår i: Astrophysical Journal. - Philadelphia, PA : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 243, s. L123-L126
  • Tidskriftsartikel (refereegranskat)abstract
    • High-resolution VLBI observations made at a frequency of 22.235 GHz of the quasar 3C 345 are discussed. Antennas located at the Crimean Astrophysical Observatory, USSR; Onsala, Sweden; Effelsberg, West Germany; and the Haystack Observatory, Massachusetts were employed at 4-min integration times to provide baselines ranging up to 5.5 x 10 to the 8th wavelengths. About 40% of the total flux density of 7.85 Jy, observed in November 1977, and 8.05 Jy, observed in October 1978, is found to originate in an unresolved component of the quasar core in a region less than 0.1 milliarcsec in diameter. The elongated jet-like component of the quasar is observed to contain several peaks of emission extending up to 6 milliarsec from the core which decreased in extent between the two observations.
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