Journals
2013 EN
Markus Kubicek · Ghislain M. Rupp · Stefanie Huber
+6 more
Cation diffusion was investigated in La0.6Sr0.4CoO3-δ (LSC) thin films on (100) yttria stabilized zirconia in the temperature range 625-800 °C. Isotopic ((86)Sr) and elemental tracers (Fe, Sm) were used to establish diffusion profiles of the cations in bi- and multi-layered thin films. The profiles were analyzed by time of flight-secondary ion mass spectrometry (ToF-SIMS). Grain and grain boundary diffusion coefficients of the cations were determined for LSC thin films with columnar grains - diffusion along grain boundaries is shown to be about three orders of magnitude faster than in grains. This could be verified for thin films with different grain size. A- and B-site cations showed very similar temperature dependencies with activation energies of ∼3.5 eV for bulk and ∼4.1 eV for grain boundary diffusion. The importance of cation diffusivities for surface segregation of Sr and thus for a major degradation mechanism of LSC cathodes in solid oxide fuel cells is discussed.
Royal Society of Chemistry
Journals
2013 EN
Jaime GarciaHartjes · Silvia Bernardi · C.A.G.M. Weijers
+5 more
Cholera toxin (CT), the causative agent of cholera, displays a pentavalent binding domain that targets the oligosaccharide of ganglioside GM1 (GM1os) on the periphery of human abdominal epithelial cells. Here, we report the first GM1os-based CT inhibitor that matches the valency of the CT binding domain (CTB). This pentavalent inhibitor contains five GM1os moieties linked to a calix[5]arene scaffold. When evaluated by an inhibition assay, it achieved a picomolar inhibition potency (IC50 = 450 pM) for CTB. This represents a significant multivalency effect, with a relative inhibitory potency of 100,000 compared to a monovalent GM1os derivative, making GM1os-calix[5]arene one of the most potent known CTB inhibitors.
Royal Society of Chemistry
Journals
2013 EN
C. Sotomayor-Beltran · C. Sobey · J. W. T. Hessels
+77 more
Faraday rotation measurements using the current and next generation oflow-frequency radio telescopes will provide a powerful probe of astronomicalmagnetic fields. However, achieving the full potential of these measurementsrequires accurate removal of the time-variable ionospheric Faraday rotationcontribution. We present ionFR, a code that calculates the amount ofionospheric Faraday rotation for a specific epoch, geographic location, andline-of-sight. ionFR uses a number of publicly available, GPS-derived totalelectron content maps and the most recent release of the InternationalGeomagnetic Reference Field. We describe applications of this code for thecalibration of radio polarimetric observations, and demonstrate the highaccuracy of its modeled ionospheric Faraday rotations using LOFAR pulsarobservations. These show that we can accurately determine some of thehighest-precision pulsar rotation measures ever achieved. Precision rotationmeasures can be used to monitor rotation measure variations - either intrinsicor due to the changing line-of-sight through the interstellar medium. Thiscalibration is particularly important for nearby sources, where the ionospherecan contribute a significant fraction of the observed rotation measure. We alsodiscuss planned improvements to ionFR, as well as the importance of ionosphericFaraday rotation calibration for the emerging generation of low-frequency radiotelescopes, such as the SKA and its pathfinders.
Journals
2013 EN
T. E. Hassall · B. W. Stappers · P. Weltevrede
+60 more
Some radio pulsars show clear drifting subpulses, in which subpulses are seento drift in pulse longitude in a systematic pattern. Here we examine how thedrifting subpulses of PSR B0809+74 evolve with time and observing frequency. Weshow that the subpulse period (P3) is constant on timescales of days, monthsand years, and between 14-5100 MHz. Despite this, the shapes of the driftbandschange radically with frequency. Previous studies have concluded that, whilethe subpulses appear to move through the pulse window approximately linearly atlow frequencies ( 820 MHz) near to the peak of the averagepulse profile. We use LOFAR, GMRT, GBT, WSRT and Effelsberg 100-m data toexplore the frequency-dependence of this phase step. We show that the size ofthe subpulse phase step increases gradually, and is observable even at lowfrequencies. We attribute the subpulse phase step to the presence of twoseparate driftbands, whose relative arrival times vary with frequency - onedriftband arriving 30 pulses earlier at 20 MHz than it does at 1380 MHz, whilstthe other arrives simultaneously at all frequencies. The drifting pattern whichis observed here cannot be explained by either the rotating carousel model orthe surface oscillation model, and could provide new insight into the physicalprocesses happening within the pulsar magnetosphere.
Journals
2013 EN
M. P. van Haarlem · M. W. Wise · A. W. Gunst
+197 more
LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometerconstructed in the north of the Netherlands and across europe. Utilizing anovel phased-array design, LOFAR covers the largely unexplored low-frequencyrange from 10-240 MHz and provides a number of unique observing capabilities.Spreading out from a core located near the village of Exloo in the northeast ofthe Netherlands, a total of 40 LOFAR stations are nearing completion. A furtherfive stations have been deployed throughout Germany, and one station has beenbuilt in each of France, Sweden, and the UK. Digital beam-forming techniquesmake the LOFAR system agile and allow for rapid repointing of the telescope aswell as the potential for multiple simultaneous observations. With its densecore array and long interferometric baselines, LOFAR achieves unparalleledsensitivity and angular resolution in the low-frequency radio regime. The LOFARfacilities are jointly operated by the International LOFAR Telescope (ILT)foundation, as an observatory open to the global astronomical community. LOFARis one of the first radio observatories to feature automated processingpipelines to deliver fully calibrated science products to its user community.LOFAR's new capabilities, techniques and modus operandi make it an importantpathfinder for the Square Kilometre Array (SKA). We give an overview of theLOFAR instrument, its major hardware and software components, and the corescience objectives that have driven its design. In addition, we present aselection of new results from the commissioning phase of this new radioobservatory.
Journals
2013 EN
S. Yatawatta · A. G. de Bruyn · M. A. Brentjens
+86 more
The aim of the LOFAR Epoch of Reionization (EoR) project is to detect thespectral fluctuations of the redshifted HI 21cm signal. This signal is weakerby several orders of magnitude than the astrophysical foreground signals andhence, in order to achieve this, very long integrations, accurate calibrationfor stations and ionosphere and reliable foreground removal are essential. Oneof the prospective observing windows for the LOFAR EoR project will be centeredat the North Celestial Pole (NCP). We present results from observations of theNCP window using the LOFAR highband antenna (HBA) array in the frequency range115 MHz to 163 MHz. The data were obtained in April 2011 during thecommissioning phase of LOFAR. We used baselines up to about 30 km. With about 3nights, of 6 hours each, effective integration we have achieved a noise levelof about 100 microJy/PSF in the NCP window. Close to the NCP, the noise levelincreases to about 180 microJy/PSF, mainly due to additional contamination fromunsubtracted nearby sources. We estimate that in our best night, we havereached a noise level only a factor of 1.4 above the thermal limit set by thenoise from our Galaxy and the receivers. Our continuum images are several timesdeeper than have been achieved previously using the WSRT and GMRT arrays. Wederive an analytical explanation for the excess noise that we believe to bemainly due to sources at large angular separation from the NCP.
Journals
2013 EN
Johan Olofsson · Thomas Henning · Markus Nielbock
+5 more
Warm debris disks are a sub-sample of the large population of debris disks,and display excess emission in the mid-IR. Around solar-type stars, very fewobjects show emission features in mid-IR spectroscopic observations, that areattributed to small, warm silicate dust grains. The origin of this warm dustcan possibly be explained either by a collision between several bodies or bytransport from an outer belt. We present and analyse new far-IR Herschel/Pacsobservations, supplemented by ground-based data in the mid-IR (VLTI/Midi andVLT/Visir), for one of these rare systems: the 10-16 Myr old debris disk aroundHD 113766 A. We improve an existing model to account for these newobservations, and better constrain the spatial distribution of the dust and itscomposition. We underline the limitations of SED modelling and the need forspatially resolved observations. We find that the system is best described byan inner disk located within the first AU, well constrained by the Midi data,and an outer disk located between 9-13 AU. In the inner dust belt, our previousfinding of Fe-rich crystalline olivine grains still holds. We do not observetime variability of the emission features over at least a 8 years time span, ina environment subjected to strong radiation pressure. The time stability of theemission features indicates that {\mu}m-sized dust grains are constantlyreplenished from the same reservoir, with a possible depletion ofsub-{\mu}m-sized grains. We suggest that the emission features may arise frommulti-composition aggregates. We discuss possible scenarios concerning theorigin of the warm dust. The compactness of the innermost regions as probed byMidi, as well as the dust composition, suggest that we are witnessing theoutcomes of (at least) one collision between partially differentiated bodies,in an environment possibly rendered unstable by terrestrial planetaryformation.
Journals
2013 EN
Ashish Asgekar · J. B. R. Oonk · S. Yatawatta
+89 more
Cassiopeia A was observed using the Low-Band Antennas of the LOw FrequencyARray (LOFAR) with high spectral resolution. This allowed a search for radiorecombination lines (RRLs) along the line-of-sight to this source. Fivecarbon-alpha RRLs were detected in absorption between 40 and 50 MHz with asignal-to-noise ratio of > 5 from two independent LOFAR datasets. The derivedline velocities (v_LSR ~ -50 km/s) and integrated optical depths (~ 13 s^-1) ofthe RRLs in our spectra, extracted over the whole supernova remnant, areconsistent within each LOFAR dataset and with those previously reported. Forthe first time, we are able to extract spectra against the brightest hotspot ofthe remnant at frequencies below 330 MHz. These spectra show significantlyhigher (15-80 %) integrated optical depths, indicating that there issmall-scale angular structure on the order of ~1 pc in the absorbing gasdistribution over the face of the remnant. We also place an upper limit of 3 x10^-4 on the peak optical depths of hydrogen and helium RRLs. These resultsdemonstrate that LOFAR has the desired spectral stability and sensitivity tostudy faint recombination lines in the decameter band.
Journals
2013 EN
M. Iacobelli · M. Haverkorn · E. Orrú
+85 more
The characteristic outer scale of turbulence and the ratio of the random toordered components of the magnetic field are key parameters to characterisemagnetic turbulence in the interstellar gas, which affects the propagation ofcosmic rays within the Galaxy. We provide new constraints to those twoparameters. We use the LOw Frequency ARray (LOFAR) to image the diffusecontinuum emission in the Fan region at (l,b) (137.0,+7.0) at 80"x70"resolution in the range [146,174] MHz. We detect multi-scale fluctuations inthe Galactic synchrotron emission and compute their power spectrum. Applyingtheoretical estimates and derivations from the literature for the first time,we derive the outer scale of turbulence and the ratio of random to orderedmagnetic field from the characteristics of these fluctuations . We obtain thedeepest image of the Fan region to date and find diffuse continuum emissionwithin the primary beam. The power spectrum of the foreground synchrotronfluctuations displays a power law behaviour for scales between 100 and 8 arcminwith a slope of (-1.84+/-0.19). We find an upper limit of about 20 pc for theouter scale of the magnetic interstellar turbulence toward the Fan region. Wealso find a variation of the ratio of random to ordered field as a function ofGalactic coordinates, supporting different turbulent regimes. We use powerspectra fluctuations from LOFAR as well as earlier GMRT and WSRT observationsto constrain the outer scale of turbulence of the Galactic synchrotronforeground, finding a range of plausible values of 10-20 pc. Then, we use thisinformation to deduce lower limits of the ratio of ordered to random magneticfield strength. These are found to be 0.3, 0.3, and 0.5 for the LOFAR, WSRT andGMRT fields considered respectively. Both these constraints are in agreementwith previous estimates.
Journals
2013 EN
P. Schellart · A. Nelles · S. Buitink
+102 more
The low frequency array (LOFAR), is the first radio telescope designed withthe capability to measure radio emission from cosmic-ray induced air showers inparallel with interferometric observations. In the first $\sim2\,\mathrm{years}$ of observing, 405 cosmic-ray events in the energy range of$10^{16} - 10^{18}\,\mathrm{eV}$ have been detected in the band from $30 -80\,\mathrm{MHz}$. Each of these air showers is registered with up to$\sim1000$ independent antennas resulting in measurements of the radio emissionwith unprecedented detail. This article describes the dataset, as well as theanalysis pipeline, and serves as a reference for future papers based on thesedata. All steps necessary to achieve a full reconstruction of the electricfield at every antenna position are explained, including removal of radiofrequency interference, correcting for the antenna response and identificationof the pulsed signal.