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A daily view of all the goings-on at ASTRON and JIVE.

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    © Image copyright David Kaplan 2014

    A group of researchers, at ASTRON and elsewhere, recently reported (daily image) the discovery and initial study of a pulsar with two white dwarf companions. This very unusual combination offers the possibility of some very interesting timing studies, including a test of Einstein's theory of gravity, but only if the system is "clean", that is, the companions really are white dwarfs. The paper this image is from studies the system in detail with optical spectroscopy to understand what the companions actually are.

    The image above shows the results. The top panel shows a detailed optical spectrum of the system, with the Doppler effect of orbital motion removed. The black points are the measured spectrum, and the red line is the best-fit model; this fitting shows the white dwarf is at 15800 K and is able to estimate its surface gravity (which tells us about its size, and combined with brightness measurements, its distance, which is about 1300 parsecs). The blue points are the spectrum of a nearby quasar used for comparison.

    The bottom panel shows the companion's velocity as measured from Doppler shifts, showing the inner companion moving around the pulsar. The inset shows that even in these velocities we can see the acceleration due to the outer companion's gravity. The amplitude of the velocity curve matches exactly that predicted from the pulsar timing. The fitting is also able to show that the system is moving away from us at 30 km/s.

    This paper is accepted to the Astrophysical Journal (in press) and is available on the arXiv.

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  • 02/17/14--16:00: Changing your profile
  • © Erwin de Blok

    One of the most important properties of a galaxy is its mass. This helps us determine, for example, how much dark matter there is in a galaxy. Radio observations are typically used for this, as the neutral hydrogen in galaxies (which we observe in the 21-cm line with radiotelecopes) is very extended, and gives us the clearest picture of the mass distribution. By measuring how fast a galaxy rotates we can then measure its "dynamical mass". Unfortunately, most galaxies are too distant for these kind of detailed observations, but we can use the "global profile" of a galaxy to still give us an estimate. A global profile shows us how much emission there is at each observed velocity in a galaxy. The width of that profile is then used as an estimate of the rotation velocity.

    The shape of the profile depends not only on the rotation velocity of a galaxy, but also on the distribution of the gas in that galaxy. In the top panel of the picture we see the same galaxy (NGC 3198) in neutral hydrogen gas (left), infra-red stellar light (middle) and molecular gas (CO; right). The hydrogen is more extended than the stars, while the molecular gas is less extended. This affects the shape of the global profile. The bottom panel shows for one model galaxy what its global profile would look like in neutral hydrogen (dark gray, widest profile) and in CO (light orange profile). Other colours show profiles for various other gas distributions. The shape and width of the profiles clearly vary significantly.

    In a paper that is available at Erwin de Blok (ASTRON) and Fabian Walter (Max Planck Institute for Astronomy, Heidelberg) model many of these profiles and do a detailed study of the impact that these different shapes have on studies of high- and low-redshift galaxies. Observations of gas in galaxies are usually done in neutral hydrogen at low redshift, but in CO at high redshift. The clear difference in profile shape, for the same galaxy, shows that one has to be careful in comparing observations of the dynamics of galaxies at low and high redshift.

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    On Friday, January 31st, Greg defended his PhD thesis entitled "Radio Frequency Interference spatial processing for modern radio telescopes" at the University of Orleans (France). This PhD was a partnership between ASTRON and the CNRS, and co-funded by ASTRON, CNRS and the Conseil General de la Region Centre.

    The committee was composed of Dr. Albert-Jan Boonstra (ASTRON), Prof. Amir Leshem (Bar-Ilan University), Prof. Asoke Nandi (Brunel University), Prof. Karim Abed-Meraim (University of Orleans), Prof. Gilles Theureau (Nancay observatory), Prof. Rachid Harba (University of Orleans), Dr. David Valls-Gabaud (Paris-Meudon observatory) and Dr. Rodolphe Weber (University of Orleans).

    After introducing Radio Frequency Interference (RFI) issues in radio astronomy, Greg presented three spatial processing techniques: interference subspace subtraction (see pictures), and the orthogonal and the oblique projection. A third section addressed the problem of interference subspace estimation. He showed the latest results obtained with the French EMBRACE station at the Nancay observatory after implementing an oblique-projection-based RFI mitigation algorithm. All techniques were illustrated with simulations, and with LOFAR LBA and HBA data, acquired with Dutch LOFAR stations.

    The pictures show Greg during his defense, and also a result of subspace-based interference mitigation. A copy of the thesis will be available at ASTRON in the coming weeks.

    Greg would like to thank all his friends at ASTRON for the help and the good times they gave him during the years of his stay. His next destination is Sydney, Australia, where he will work with Dr. Aaron Chippendale and Prof. Brian Jeffs on RFI mitigation with phased array feeds.

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    © Monika Moscibrodzka

    The Galactic center is an ideal laboratory for testing various theoretical models of the magnetized plasma flows onto compact objects. I will present highlights of general relativistic magnetohydrodynamic simulations of gas falling onto a black hole. Our models predict dynamical and radiative properties of hot accretion flows around a Kerr black hole. I will show how the Sgr A* observational data put constraints on the model parameters such as viewing angle, spin of the black hole or mass accretion rate. Also, the recent discovery of a cloud moving towards the Galactic Center creates a potential opportunity for testing models of Sgr A*. I will show one of the possible scenarios for the evolution of flux and size of the radio source in such an event of enhanced mass-accretion rate. Finally, I will discuss the current models limitations and prospects for detecting the black hole silhouette.

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  • 02/20/14--16:00: Low Noise Tile
  • © copyright ASTRON

    The Low Noise Tile (LNT) project aims to demonstrate state-of-the-art noise performance of aperture arrays for the mid-frequency range (MFAA) of the SKA. A recent AJDI of December 18, 2013 showed the first measurements of the noise temperature of the new LNT frontend design based on the integrated SkyWorks LNA. This gives a considerable improvement over the previous one, which used using a discrete Avago transistor, and was shown in the AJDI of August 6, 2010. The AJDI of last December showed results of connectorized frontend boards/LNAs, with the promise of improved noise performance for an aperture array system.

    Today, as a next step, we present a similar improvement for the new LNA, now integrated with the Vivaldi antenna element. The noise temperature was measured in a special single-antenna test fixture in THACO (for THACO see the AJDI of July 11, 2011). While the previous design gave a noise temperature of approximately 60 K in the test fixture, this has decreased to approximately 40 K for the new antenna element. The improvement gives a good indication of what may be achieved for an array of these new elements.

    The images show the single antenna test fixture used for the measurements, the integrated antenna/frontend board, and a plot of the noise temperatures as a function of frequency for the "old" and the new LNT elements in the antenna test fixture.

    We are now in the process of producing more new elements to place in the array, so we can directly compare the array performance using the "old" elements with that of the array with the improved LNT elements. Stay tuned for results of a 2x2 array soon, and results for a completely filled (~ 72 elements) array later.

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    © Madroon Community Consultants (MCC)

    Last week, a mildly ecstatic crowd of starstruck locals, and a smattering of awestruck others(*), filled the Van de Hulst auditorium to watch the Dutch rule the speed-skating in the Sotsji Olympics. They had been lured to the big screen to avoid the network being overloaded by individuals watching the event from their offices. Many had brought their little laptops, partly because they would feel naked without, but also because speed-skating can get a little boring for those with the wrong kind of mirror neurons.

    Although it is nice to head the Olympic medals table, the crushing dominance of the Dutch speed-skaters is a mixed blessing. It makes us look a bit silly to pour limitless passion and resources into a sport that is mesmerizing at best, but does not sport any marketable features like sexy outfits, or the thrill of grievous bodily harm. This 10km event followed the usual routine: records were broken and the winner's pedestal was uniformly orange. The only excitement came from the fact that the wrong Dutchman unexpectedly won.

    Foreign commentators try to explain the phaenomenon by conjecturing that skating is the only way to get about in winter in the Netherlands. If only that were true; it would be a welcome change from clonking around in our clumsy wooden shoes. In fact, there has been no ice at all this year, even though the rapid disappearance of the Arctic sea ice is deemed to give us colder winters for a few decades.

    Nevertheless, skating is deeply ingrained in a country that has lots of canals, and where it just gets cold enough in winter for the water to freeze, with very little snow to ruin the black ice. Of course, the entire ASTROJIVE family gladly joins in the fun whenever possible. Just search our AJDI archive with the keyword "skating" (but not "skate").

    (*) Throughout the event, a noted Glaswegian was twittering a blow-by-blow account to a breathless world.

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    © ASTRON

    On February 19, our former colleague Lambert Nieuwenhuis passed away at the age of 67. Lambert became very ill shortly after his retirement. Because of his good physique and excellent condition, he could fight his illness successfully for a long period of time. Recently, however, physical complications increased, and we were shocked to hear that he was rapidly losing the unequal fight. We can only feel sad and dejected that he passed away so soon.

    Lambert was a dedicated colleague who excelled by his thoroughness and reliability. He had a sharp eye for high quality in his work. This way he made significant contributions to many projects, in particular the WSRT receiver systems. He commanded respect due to his perseverance, which showed especially in his activities in the workers council but also in cases where he felt injustice, personally or otherwise. Lambert was very creative and made drawings in his spare time. People admired the drawings of his sons which hung in his office.

    Lambert was a keen sportsman and cycled to his work every day. He was also a devoted runner who ran on a weekly basis with colleagues and other "Dwingelers" He completed the marathon of Rotterdam and participated in the Cascade run in Hoogeveen every year. Doing so, he built the iron condition that helped him for a long time in his battle against the illness he knew he would lose in the end.

    We will remember Lambert as a sportive and dedicated colleague. Our thoughts are with his family and friends.

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  • 02/25/14--16:00: LOFAR HBA repair
  • © ASTRON

    The daily image of February 10th showed a severely storm-damaged LOFAR high band antenna (HBA) tile, that had to be transported carefully to a suitable barn. It has now been taken apart, and almost 60% of the polystyrene had to be replaced. The result was that we almost had to build a completely new tile.

    The security guards of the WSRT gathered the spare parts from the warehouse near the WSRT, and lent a hand in the reconstruction. We learned a lot about optimizing the repair process. The stepwise procedure we developed can also be used for the production of small series of new HBA antenna tiles.

    The picture shows the flexibility of our WSRT security staff, involved as they are in a totally different activity outside their normal work.

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  • 02/27/14--16:00: Ghostbusted
  • © Rhodes University, SKA SA & ASTRON

    Selfcal "ghosts" are a mystery with a long ASTRON heritage. They were first spotted by Ger de Bruyn in a 2004 WSRT 92cm observation (bottom panel above). The ghosts were a line of source-like and (mostly) negative artefacts, arranged along a line linking the brightest source in the field (bottom right) with Cyg A (top left), which was coming in through a sidelobe about 20 deg away. Some of their more fascinating features -- the regularity of the pattern, the PSF-like sidelobes that were nonetheless substantially different from ghost to ghost, etc. -- caused a lot of discussion at the time. But with no adequate explanation presenting itself, they seemed destined to be forgotten as yet another one-off interferometric artefact, but for Jan Noordam keeping a slowly yellowing printout of the map pinned to his whiteboard for the next six years. (Incidentally, this may also explain the appearance of ghosts at his retirement party: )

    In 2010, during Ger and Oleg Smirnov's Quality Monitoring Committee project, the pattern emerged again. This time, we could empirically reproduce the ghosts using MeqTrees simulations. It quickly became apparent that ghosts were associated with missing sources in the sky model, and that they somehow seemed fundamental to the selfcal process. The exact mechanism, however, remained unclear, and neither could the puzzling features be explained.

    Thanks to some recent work at Rhodes University and SKA South Africa, the puzzle is now solved, at least for WSRT. Ghosts are indeed due to calibration solutions trying to "compensate" for missing model sources. We now have a theoretical framework for predicting their emergence (to which earlier work by Stefan Wijnholds and Alle-Jan van der Veen contributed some crucial insights). The mathematically-inclined are encouraged to indulge in a recently accepted paper (

    The top panel above summarizes the theoretical predictions in a simple two-source sky, one-source model scenario (where the missing source is at +1 deg). After calibration, each baseline "sees" its own set of ghosts; these are arranged at fixed intervals inversely proportional to baseline length, with varying amplitudes. The plot shows the positions and amplitudes of these ghosts for a selection of baselines. Baseline 0D yields the shortest intervals, while 9A yields the longest. Note that redundant baselines (here, 01 and 12, 05 and 16) produce ghosts at the same positions but with different amplitudes (sometimes even of opposite sign!). Due to WSRT's highly regular configuration, some positions end up hosting ghosts from many baselines, and it is there that we ultimately see the strongest ghosts in the resulting maps. The "ghost spread function" at each position is determined by which baselines actually contribute to it, and therefore varies from ghost to ghost. The negative ghost at 1 deg (sitting on top of the unmodelled source, negative on all baselines and thus producing the strongest overall response) is actually routinely known under the guise of "flux suppression". The remarkable thing about this plot is how a relatively simple mathematical process can yield such a rich variation!

    It turns out that ghosts have always been with us, quietly sitting under the noise in all observations. This is a worry for future deep surveys and statistical detection techniques, and must be understood further. For 2D arrays, the mechanism becomes more complicated (and the pattern less regular), and we're currently working to explain it. Ger has recently spotted a ghost at the -1 ("anti-source") position in LOFAR observations, and so has the LOFAR Transients KSP. Clearly, Jan needs to keep that old map pinned up for a while yet.

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    © John Cannon

    The ALFALFA blind extragalactic HI survey has cataloged tens of thousands of gas-rich galaxies in the local universe and has populated the faint end of the HI mass function with statistical confidence for the first time. In this talk I will present results from comprehensive follow-up observing campaigns to study the low-redshift, low-mass, gas-rich dwarf galaxy population discovered by ALFALFA. The centerpiece of this effort is the Survey of HI in Extremely Low-mass Dwarfs (SHIELD). SHIELD is an ongoing multi-wavelength investigation of the properties of 12 extremely low-mass galaxies selected from an early ALFALFA data release. I will present SHIELD results from imaging and spectroscopic observing campaigns with the VLA, HST, Spitzer, GALEX, and WIYN telescopes. I will then discuss results from parallel ongoing observing programs that fully exploit the faint end of the galaxy mass function derived by ALFALFA. These efforts include surveys of "ultra compact high velocity clouds" (HI clouds with structural parameters that match those of gas-bearing "mini-halos" if located within ~1 Mpc), candidate "dark galaxies" (systems with extreme hydrogen mass to stellar light ratios), and targeted studies of individual sources of interest (including the extreme galaxy Leo P, one of the most metal-poor galaxies known in the universe). Taken as a whole, these observing programs are furthering our understanding of the continuum of galaxy properties in the extremely low-mass regime.

    Image: The Extreme Star-Forming Galaxy Leo P - a dwarf galaxy recently discovered by the ALFALFA survey. Subsequent observations have revealed that it is one of the most extreme galaxies known in the universe. This color optical image of Leo P is overlaid with contours showing the locations of neutral hydrogen gas.

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    © Andre van Es / Jan van Muijlwijk

    They say that young lovers are "over the Moon" with happiness. Last Valentine's Day (14 feb 2014), our Andre van Es and Marit Gant took this almost literally by allowing the momentous words "I do" to strike home via the the silvery face of the Goddess of Romance. As a bonus, the Moon itself threw in a special blessing.

    Things were done by modulating a powerful radio beam with the marital vows, and bouncing it off the Lunar surface(*), 380.000 km away. After demodulation, the words came back loud and clear, leaving zero room for doubt as to their intention. The 2.5 sec delay was entirely due to the travel time to the Moon and back, and not to any hesitation on the part of the newlyweds.

    As far as we know, this is a first. It was an opportunity for the members of CAMRAS(**) to express their appreciation for their Chairman. The ceremony was officiated over by Moon-bouncer Jan van Muijlwijk, under the benevolent smile of ASTRON Director Mike Garrett.

    See also the charming video:

    (*) This technique is also used, with somewhat larger radio telescopes and more powerful transmitters, to bounce signals off nearby asteroids, and even the planet Venus.

    (**) The C.A.Muller Radio Astronomy Station (CAMRAS) is an organisation of radio enthousiasts who are using the venerable 25m radio telescope on the edge of the Dwingeloo heath for scientific experiments and outreach. More information may be gleaned by clicking on "archive" at the top of this webpage, and searching with the keyword "CAMRAS".

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    © Astron

    On February 19th, ASTRON Managing Director (and former Head of the R&D lab) Dr Marco de Vos has been invested as parttime professor of Computer Science and Sensor Systems at the Hanze Institute of Technology (HIT) in Assen. To mark the the occasion, he delivered a highly inspiring inaugurational lecture on his plans for the research to be be carried out on sensor technology and ICT at the Hanze University(*).

    With the help of some illuminating examples he emphasized the importance of understanding how and what we measure with our sensors. Only then can the next step be taken towards knowledge and control. Marco stressed that we are on the brink of the next ICE-age, where ICE stands for Information, Communication and Energy.

    In his speech, the Dean of the Faculty characterized Marco as a person with "tons of enthusiasm", an observation that will be easily recognized by many of us in radioastronomy. Congratulations, Marco, and we wish you tons of satisfaction and success!

    The institute takes its name from the important Hanze trading network, which extended over a large part of NW Europe in the late Middle Ages. It was one of the roots of the wealth and power of the subsequent Dutch Republic (1579-1795).

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    In a message to EVNtech, Cristina Garcia Miro, our VLBI friend at the Robledo NASA DSN station, writes:

    "Coinciding with the 50th Anniversary of NASA's Deep Space Network we are very glad to announce the demonstration of the first fringes between the new DSN VLBI digital backend (DVP) and the European VLBI Network (EVN).

    The DVP, an in-house JPL development using CASPER/Berkeley technology and the Mark5C Haystack recorder, provides the impressive sensitivity of 19.5 uJy in a 70m-70m DSN baseline (18cm, 1-sigma 150min integration) and is contributing to make the Deep Space Network a world-class Radio Astronomy instrument."

    The figure shows the fringes (at a wavelength of 18cm) between the DSN Robledo telescope (70m) in Spain and the EVN Effelsberg telescope (100m) in Germany. The data were correlated with the EVN SFXC correlator at JIVE in Dwingeloo, when Cristina visited us last January.

    For news about the 50th DSN Anniversary check .

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    © Anthony Rushton

    Relativistic feedback from compact objects appears to play a major role in the evolution of galaxies, but understanding the accretion conditions that launch jets still remains poorly understood. Nearby compact sources can provide a test bed for studying how accretion disks can launch jets at very different fractions of the Eddington rate. I will present results from X-ray binaries (XRBs), Ultra-luminous sources (ULXs) and the Galactic Centre (Sgr A*), that shows jet production varying over a time scale of a minutes to months; this work provides insight to underlying mass accretion rates and can be used to estimate radiative efficiency of the bolometric luminosity from black hole candidates. Ultimately, with a good calibration of the disk-jet relationship, ALMA and SKA will eventually be able to trace the black hole mass-function in the local universe.

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  • 03/06/14--16:00: A scintillating radio movie
  • © Ger de Bruyn

    The two movies above each show a series of 720 images, 6 hours at 30s time resolution, of LOFAR high-band observations of the same EoR-3C196 field, taken in December 2013. The movies are 600 times faster than real time. Both show the same field of view (2.5x 2.5 deg) with a 3' PSF. If you stare at them for a while you will notice that the right images are rather 'unstable', as compared to those on the left. So what is causing the difference?

    We are all familiar with optical seeing: the intensity (and position) fluctuations of stars due to light propagation through the turbulent neutral troposphere. It is also called scintillation. This phenomenon can also occur at radio wavelengths where it is caused by turbulence in the ionized plasma between source and observer: the Galactic interstellar medium, the interplanetary medium and the ionosphere. Ionospheric scintillation is a relatively rare phenomenon. If it occurs, it is mostly in the late evening to midnight hours, but it can persist for many, many hours. During LOFAR Cycle-0 observations, ionospheric scintillation was relatively rare, so we got spoiled. However, since November 2013 we see a marked increase in the frequency of ionospheric scintillations, probably related to enhanced Solar activity during the secondary maximum in this Solar cycle. Most discrete radio sources will exhibit the phenomenon and can easily exhibit intensity variations of 20% (rms) at 150 MHz, accompanied by large image motion. Both effects combined make it impossible to make high dynamic range images and these data will therefore not be useful for the primary goals of the LOFAR EoR project.

    The movies give us fascinating insights into small-scale ionospheric phenomena. During periods of scintillation the lateral scale of significant (say 1 radian) phase fluctuations in the ionosphere approaches the projected Fresnel scale at the height of the ionosphere which is a few km at 2m wavelength. The wavefront then gets wrinkled so much that different parts of the wavefront interfere (either constructively or destructively) on the ground. Both the intensity and the apparent position of the source then vary.

    Some technical details on the making of the movies:

    To avoid that the images are dominated by the side-lobes of 3C196 - an 80 Jy source - we have peeled (= self calibrated and subtracted) it from the data. The residual visibilities were imaged using Sarod Yatawatta's impressive ExCon imager. Snapshot images were combined into a movie using scripts created by V. Pandey and Vibor Jelic. By using only the core stations, yielding a resolution of about 3', the snapshot images could be made without selfcalibration (which would be a real challenge anyway !).

    To reduce the array side-lobe pattern, a total of 10 sub bands (of 0.18 MHz each), spanning the frequency range from 115-160 MHz, were combined. During the periods of very rapid image motion (right movie) the images become blurred. This is partly because ionospheric refraction scales inversely with the square of the frequency. The careful observer can also see the side-lobes of the brightest sources rotate around them during the 6 hour synthesis time.

    During some of those periods, the signals of 3C196 become decorrellated, and very faint (1-2%) residuals of 3C196 are then visible in the lower left corner (at 08h13+48d13'). At least a dozen sources can be detected in the image, with flux densities ranging from 1 - 4 Jy. The sources show mostly uncorrelated movements and intensity fluctuations. There is obviously a wealth of information in these movies on the motions and turbulence in the ionosphere on angular scales down to 30', which corresponds to linear scales of 3 - 4 km in the ionosphere at an altitude of 300 km.

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    © Paul Boven

    Last week, JIVE Director and parttime rock star Dr Huib Jan van Langevelde delivered his oratio, i.e. the public lecture that marks the inauguration of a Professor at a Dutch university. He is to teach Galactic Radio Astronomy at Leiden University, which used to be his Alma Mater in days of yore.

    His theme was Clarity: Brightness and Transparency in Radio Astronomy. He started with a clear exposition of galactic astronomy for the benefit of the non-professionals in the festive audience. And of course he brightly discussed the remarkable things one can do with Very Long Baseline Interferometry (VLBI). But, being the Director of an important international institute, he also took the opportunity to lob a few policy ruminations over the heads of the audience at the various agencies that finance large European science infrastructures. In essence, he urged them to be more transparent in their dealings, for instance with the European VLBI Network (EVN). The entire text may be found at

    Afterwards, there was the usual gathering of the Dutch astronomical family, raising their glasses to the young Prof. Among other luminaries was Richard Schilizzi, the visionary creator and first Director of the Joint Institute for VLBI in Europe (JIVE).

    More information may be gleaned by clicking on "Archive" at the top of this webpage, and searching with the keyword "VLBI" or "JIVE". Or, alternatively, by means of the following link.

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    © ASTRON

    The previous daily images (17-06-2013 and 09-09-2013) showed the four environmental prototypes inside the WSRT bouwhal, and the anchor test performed at the South Afrika Karoo SKA site. Since then, the dissipating electronics and sensors for temperature and humidity were added and tested. This all took place at chilly temperatures in the Netherlands. All worked satisfactorily, so the prototypes could be taken apart and loaded into the 40ft sea container to be shipped to its warm destination. The transfer from the cold Northern hemisphere to the south is done by the ocean vessel RED CEDAR 4207 shown in one of the pictures. You can check out the current position of the RED CEDAR at the marinetraffic site

    The goal is to place four different MFAA prototypes in the field for several years. This allows the environment to "decide" which one is most suitable. The idea is to incorporate as many different design features as possible in these few carefully defined prototypes. All prototypes have varying levels of protection. Two of them have only a cover for rain and Sun, while the delicate electronics are protected locally by special coatings. Another prototype resembles the LOFAR HBA with its insulated housing, made of packaging material. The fourth is fully closed and made of low-cost plastics.

    These environmental prototypes are part of the Array Prototypes work package of the

    SKA Mid-Frequency Aperture Array consortium

    This project is part-financed by the Northern Netherlands (SNN), the European Regional Development Fund, and the "Peaks in the Delta" program of the Dutch Ministry of Economic affairs, Agriculture and Innovation.

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    © LOFAR

    Members of the LOFAR cosmic ray key science project (RU Nijmegen and Groningen), with the helpful support from Menno Norden and Klaas Stuurwold (ASTRON), have installed a weather station at the concentrator node, close to the LOFAR superterp. As cosmic rays generate "air showers" of secundary particles in the atmosphere, the atmospheric conditions are important factors for the understanding of the data.

    Especially interesting is the measurement of the atmospheric electric field, which allows an independent confirmation of thunderstorms detected with LOFAR. Also, solar activity can be detected, as happened already during the first days of operations (see inset).

    Data will be taken at intervals of a minute, and will also be made available to other interested users of LOFAR. These measurements will help to define good observing conditions for the various LOFAR programs, and support the study of the performance of hardware as function of environmental conditions.

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    © Dan Stinebring, Stelios Kazantzidis

    Today's colloquium by Dan Stinebring will discuss how ASTRON researchers and colleagues around the world are using pulsars to try and detect gravitational waves from orbiting supermassive black holes. The picture of the day (produced by astronomer Stelios Kazantzidis, Ohio State University) shows a computer simulation of two Milky Way sized galaxies colliding. The supermassive black holes at their center end up orbiting each other and producing gravitational radiation that should be detectable through high-precision timing of millisecond pulsars, as Stinebring will explain.

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  • 03/13/14--17:00: M15A pulse profile
  • © Aard Keimpema

    Recently a phased array mode has been developed for SFXC, the EVN software correlator. In this mode, the signals from all stations in an experiment are summed coherently rather than correlated. The main applications are time-domain pulsar studies such as pulsar timing, and pulsar searching experiments. Compared to single dish observations, the increased signal-to-noise obtained from the phased array mode will yield higher precision timing information, and allows for the detection of weaker pulsations.

    Before the signals from all stations can be summed coherently, phase and amplitude calibration solutions need to be provided. We obtain these by first correlating the experiment like a standard VLBI experiment. The usual calibration steps are then performed in the data reduction package AIPS. The results are exported back to the correlator where they are applied during the phasing up. The resulting time series are stored in the PSRFITS data format which is widely supported by pulsar toolkits such as SIGPROC and PRESTO.

    The field of view in a dataset obtained using the phased array mode is equal to the synthesized beam of the observing array, which in VLBI is very small. To make efficient pulsar searches possible, a multiple phase centre capability is present in the phased array mode. In this mode SFXC will, in a single pass, produce a search mode data set for each requested phase centre. Enabling multiple phase centers increases processing time by approximately 50% but each phase center comes at very little additional cost.

    In the animation we show a series of folded pulse profiles of pulsar M15A, which has a period of 110.66ms and DM=67. Each profile is the result of a coherent sum of a different collection of stations. The participating stations were Arecibo(Ar), Effelsberg(Ef), Greenbank(Gb), Jodrell Bank(Jb), Onsala(On), Torun(Tr), and single dish Westerbork(Wb1). The signal to noise in the resulting profile scales linearly with the total effective collecting area. The observational data that were used to produce these plots are courtesy of Franz Kirsten (Argelander Institut f�r Astronomie) and Wouter Vlemmings (Chalmers University of Technology / Onsala Space Observatory).

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