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

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    © Kapteyn Institute/Astron

    NGC 4203 is a nearby lenticular galaxy surrounded by an unusually large HI disc. In a paper now accepted for publication in MNRAS, a team lead by a PhD student Mustafa Yildiz (Kapteyn Astronomical Institute) used the WSRT to study this large gas reservoir. They find that the gas system (shown in blue in the above image) consists of two separate components: an inner star-forming ring and an outer HI disc. The inner ring contains metal-rich gas and a large amount of dust (red in the image). In contrast, the outer disc is likely to be poor in metals and dust. While the star-formation efficiency in the inner HI ring is comparable to that typical of the inner regions of spiral galaxies, it is much lower in the outer disc -- even in regions with high gas density. Nevertheless, the star formation efficiency in the outer HI disc is still consistent with that of the outer regions of spiral galaxies. These differences might be explained with different gas origins such as stellar mass loss for the inner regions and accretion from the intergalactic medium for the outer disc. The deep optical image also reveals a dwarf galaxy interacting with NGC 4203. Data for this study were collected with the WSRT and CFHT telescopes.

    You can find more details in Star formation in the outer regions of the early type galaxy NGC 4203, Yildiz, M.K., Serra, P., Oosterloo, T.A., Peletier, R.F., Morganti, R., Duc, P.-A., Cuillandre, J.-C., Karabal, E. 2015, MNRAS, in press http://arxiv.org/abs/1505.02981


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    © Megan Argo, Ilse van Bemmel

    A team of radio astronomers, led by Megan Argo of the Jodrell Bank Centre for Astrophysics, and JIVE astronomer Ilse van Bemmel, has for the first time found evidence of the onset of a new phase of nuclear activity. The activity is associated with the central black hole in the polar ring galaxy NGC 660 (right image).

    First indications of a change in NGC 660 were found during an Arecibo survey in 2012. NGC660 increased significantly in radio brightness, but the cause was unknown. The team led by Argo and van Bemmel has combined new observations from the Westerbork, e-MERLIN and EVN radio telescopes with archival results from ground- and space-based telescopes. All evidence points to a new activity phase being triggered.

    The EVN data (left image) reveal a new, bright radio source at the location of the black hole. It has the typical core-jet structure associated with nuclear activity. The e-MERLIN observations show a spectral energy distribution typical for a very young active nucleus. Compared to observations from before 2010, the core of NGC 660 is several hundred times brighter, but the latest observations show that it is fading and may disappear over the next decade.

    The Westerbork observations are used to study the kinematics of neutral hydrogen along the line of sight to the new radio source. The results indicate there is cold gas close to the center, a potential reservoir of fuel for this new phase of nuclear activity.

    The paper: A new period of activity in the core of NGC 660, M.K. Argo, I.M. van Bemmel, S.D. Connolly & R.J. Beswick, MNRAS, in press, arXiv 1508.01781

    Also see the JIVE press release


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    © Kris Zarb Adami

    During the last week of May 2015, the AAVS1 design team (and two SKAO members) joined efforts on the beautiful island of Malta, to attend the AAVS1 Detailed Design Meeting, hosted by the university of Malta.

    The Aperture Array Verification System (AAVS1) will verify the end-to-end signal path of the Low Frequency Aperture Array (LFAA) from antenna to the output of the station beamformer. AAVS1 shall include all the components required to receive low frequency electromagnetic signals and to correlate the beams formed. LFAA is a major system and part of the Low Telescope which will be constructed in Western Australia at the Murchison Radio Observatory (MRO) as part of the first phase of the Square Kilometre Array (SKA) Observatory. AAVS1 will also be built at the MRO, at a separate location near to the SKA1_LOW Telescope.

    AAVS1 will comprise a main array (256 elements), representative of an SKA1_LOW station, and additional hardware, software and infrastructure to verify the functional and performance characteristics of this main array. In particular, the MWA telescope (including MWA post-processing pipelines) will be used for verification of the main station. Also, three auxiliary AAVS1 stations (48 elements each) will be constructed for more detailed system verification.

    AAVS1 shall include all the components required to receive low frequency electromagnetic signals and to cross-correlate the beams formed by the main station and auxiliary stations.

    Our next major milestone is the AAVS1-SRR, 2nd July 2015.


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    © Jaap van 't Klooster and ASTRON

    Last week, a great friend of Radio Astronomy (and ASTRON) left ESTEC for his third career, as parttimer in the Antenna Group of the Eindhoven University. The picture shows Kees van het Klooster at the goodbye party given by friends and colleagues, in the presence of his family.

    His achievements include the successful efforts toward the technology development of the panels of the European ALMA antennas, aspects of the ESA Herschel and Planck satellites, and fostering good relationships between scientists of west and east.

    Reference was also made to his sunny disposition and his many wordplays ("Kees Vocubulary"), of which Miss Understanding was perhaps the most innocent.

    We at ASTRON know Kees for his work on the antennas of the radio astronomy satellite QUASAT in the eighties. This covered feeds and phased arrays for space and groundbased radioastronomy, VLBI, technical workshops, not to mention many related ideas, all pushed forward with unrelenting enthousiasm.

    All the best, Kees. It has been a pleasure to work with you!


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    © ASTRON & NASA

    This afternoon around 1:49 pm (11:49 UTC), the New Horizons spacecraft will make its closest approach to Pluto and its system of moons. Unfortunately, the first images from the fly-by are not expected until later in the day but NASA TV will be following the count down to the closest approach with the mission operations team at John Hopkins University.

    We plan to present the live NASA TV stream in the van der Hulst Auditorium for those that are interested (starting around 1.30 pm - just before the closest approach). The latest images of Pluto from yesterday's pre-flyby observations are also expected to be shown around that time.

    The radio signals from the 2.1m X-band communications system (see image above) take ~ 4.5 hours to reach the Earth. Due to the large distance and the weakness of the signal (the transmitter power is only ~ 12W), the current data rate is restricted to ~ 2000 bits/second - it will take months before all the data are returned to Earth.


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    © RTV Drenthe, R.G.B. Halfwerk

    After weeks of programming, testing and integrating of numerous pieces of delicate LOFAR hardware like 192 Receiver Units (RCU), 24 RSP boards (Remote Station Processing board), 12 TBB’s (Transient Buffer Boards) , LCU’s ( Local Control Units), Network Equipment, Rubidium clock, Sync Optics boards, Line Control Panels, Power Supplies and tens of UTP, coax, infiniband and power cables, the ILT container as intended for the first of three new Polish LOFAR stations in row, station Baldy near Olsztyn has received first (led-)light when Menno Norden, System Engineer of ASTRON, entered the magic commands into the LCU!

    The regional TV station RTV Drenthe recorded this magic moment on June 24th.

    This container fully tested and ready to operate, now will be shipped to its destination to the Baldy station site in Poland where the actual roll-out is in full progress! The station Baldy is about 1.100 km from the central core of the International LOFAR Telescope (ILT)

    The Polish LOFAR consortium POLFAR will get in total three new antenna stations in the north, west and south of Poland. These will be located in Lazy (in southern Poland, operated by the Jagiellonian University in Krakow), Baldy (in northern Poland, operated by the University of Warmia and Mazury in Olsztyn), and Borowiec (in western Poland, operated by the Space Research Centre of the Polish Academy of Sciences). The POLFAR consortium consists of 10 Universities and Knowledge Centers.

    Probably you’ve noticed but a dedicated team so far has done a great job to bring again all required documentation up to date, let manufacture thousands pieces of hardware and bring all this gear together to create a new LOFAR station in row. Credits to, in random order: Albert van Duin, Albert Wieringh, Anne Koster, Edwin Stuut, Eim Mulder, Zabet Ahmadi, Han Wessels, Germon Offereins, Gijs Schoonderbeek, Henk Bokhorst, Henri Meulman, Jan Idserda, Jan Rinze Peterzon, Jan-Pieter de Reijer, Jűrgen Morawietz, Klaas Stuurwold, Marchel Gerbers, Marco Drost, Mark Ruiter, Martijn Brethouwer, Menno Norden, Michel Arts, Michiel Brentjens, Nico Ebbendorf, Patrica Breman, Peter Gruppen, Pieter Donker, Raymond v.d. Brink, Renate van Dalen, Ruud Overeem, Sieds Damstra, Sjouke Kuindersma and Teun Grit.


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    © ASTRON

    From Monday June 29th to Friday July 3rd ASTRON hosted the second all-hands meeting of the SKA Science Data Processor consortium. More than 70 people, representing over 30 institutes and companies from 9 countries gathered to discuss the next phase of the SDP project.

    This meeting was just after our Preliminary Design Review and the main focus was on clarifying the schedule and work for the next phase: preparing for delta-PDR and CDR.

    While work is important, it is equally important to gather in an informal setting to discuss the finer things in live while enjoying a cool drink. The weather made this and the bicycle rides from the various hotels and restaurants in the area even more enjoyable. Overall a very succesful meeting.


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  • 07/20/15--17:00: LOFAR sees lightning
  • © ASTRON

    The movie shows an image of the entire sky using only the super-terp LOFAR stations. Data from one 7 minute snapshot of the MSSS HBA survey were used to generate a movie frame every 3.5 seconds. The integration time of MSSS is 2 seconds, and in each frame multiple airy patterns can be seen, across the entire sky.

    The KNMI archive shows the meteorological conditions above the Netherlands at the time of the observation. We can see that a large storm was in progress, with multiple recorded lightning events. Since they are in the near field of LOFAR, the discharges (which generate radio waves) show up as de-focused radio sources. Multiple events are visible in the image; since a lightning lasts a fraction of a second, LOFAR detects many of them in 2 seconds of integration. Some events persist in the image for longer than a second.

    A time-frequency view of one super-terp baseline shows the events; they are detected on all the baselines of the observing run, varying in intensity.

    Much can be deduced from data-sets like this. Paralactic determination of the distance to the point in the cloud which generated the lightning is possible (by near-field imaging for example) and consequently a 3D model of the storm front can be produced based on that distribution. Details about the physics and how it is related to what an interferometer detects are also intriguing; are we seeing single events, or multiple reflections of a single one? Or, do we detect the ionization trail of the lightning events? LOFAR is capable of higher time resolution observations. Correlating and imaging those data sets may be of use. Also, our data archives certainly contain more events like the one presented here and are worth exploring.

    LOFAR is a multi-faceted radio astronomy toolkit with many possibilities we can exploit.

    Credit: Aleksandar Shulevski, Wilfred Frieswijk, the LOFAR Science Support group, Michiel Brentjens, the MSSS team (Heald et al. 2015)


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    © JvL

    We are glad to announce that on July 7, 2015, the Apertif Radio Transient System (ARTS) passed its Preliminary Design Review (PDR). ARTS extends Apertif for high time resolution -- for pulsars and fast radio bursts (FRBs) -- and for high angular resolution (VLBI). After a thorough review of the conceptual designs, and a one-day meeting in Dwingeloo filled with presentations by and questions to the ARTS team -- shown on the cake, above -- the international panel concluded that

    "The science cases are extremely compelling:

    Pulsar timing and VLBI provide important legacy work &

    The FRB searches are ground breaking."

    "There's a good team in place to deliver the system, given the necessary support. No obvious bottlenecks have been identified; it was clear that the team already identified the problem areas."

    So excellent preliminary feedback, overall. We now move forward with the detailed design, the CDR, and the upcoming deployment. Through its unique combination of sensitivity and wide field of view, Apertif/ARTS will arguably be the most powerful fast-transient machine in the world.


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    © DESP

    Engineering is about pushing the envelope. With UniBoard2 we wanted to break the Tbps barrier(*), and that is exactly what we have done.

    During a visit by Peter Schepers from Altera Inc. and Karl de Boois from EBV, the nitty-gritty details of their transceivers where explained to us. Within a week after that, we breached the 1Tbps barrier. This was done(**) by using twenty-four 10Gbps transceivers on the backplane side of the board for each FPGA (making in total 960 Gbps full duplex), and twenty-four optical interconnections on the front side per node (making in total 960Gbps). Although we were the first to use the Arria10 FPGAs from Altera, we were able to achieve error rates smaller than 1E-13 (one error per 10 Tbits). Last week, we were able to run all 96 transceivers for a single node (almost 1Tbps per node). This will be the target for all FPGAs on the production boards by the end of the year.

    From left to right, the victorious team consists of: Leon Hiemstra (firmware engineer Astron), Karl de Boois (Field Application Engineer EBV), Gijs Schoonderbeek (hardware design Astron), Peter Schepers (transceiver specialist Altera) and Jonathan Hargreaves (firmware engineer JIVE).

    (*) One Terabit per second (Tbps) is 1.000.000.000.000 bps, or 100.000 HDX 1080p video streams. The aggregated traffic on all AMS-IX (Amsterdam Internet Exchange) connected network ports has a peak of 3.7 Tbps, which can be handled by a single UniBoard2 when all 384 transceivers are used.

    (**) We do not apologize about the jargon. It just says it all.


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    © Bassa/Sipior

    After a half-year design and tender process, the DRAGNET GPU cluster has now been installed by ClusterVision at the Smitsborg Rekenhal in Groningen... only an arm's length away from LOFAR's COBALT correlator, which will feed DRAGNET with data at a rate of 50-200 Gb/s.

    DRAGNET features 23 worker nodes, which together house 92 state-of-the-art Titan X GPUs (each 2U node is packed with 4 Titan X's!). These consumer-grade GPUs are both cheap and powerful, each providing also 12GB of RAM, which is crucial for correcting for dispersive delay at LOFAR's low frequencies. The massive cumulative compute power of the DRAGNET cluster (~600 TFlops) will be used for pulsar and fast transient searches.

    In some observing configurations, we aim to search the data in real time, in order to enable a higher on-sky time through commensal observations and in order to trigger on interesting astrophysical events. Ultimately, we want to handle 100 tied-array beams for real-time single-pulse searches. This already puts DRAGNET within a factor of a few of what will be needed for similar science with SKA-Low! Commissioning of the cluster is underway, with significant help from Mike Sipior. Many thanks also to Arjen Koers and Teun Grit for their help with the network connections.

    The DRAGNET cluster is funded through a Starting Grant from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement nr. 337062 (PI Hessels).


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    © Pieter Benthem

    On the (early! morning of the) 2nd of July 2015, we organised the System Requirements Review of the SKA Aperture Array Verification System (AAVS1). To save people the burden of traveling, we indulged in the interesting challenge of hosting a video-conference. Teams from the UK, Italy, the Netherlands and Australia joined the call.

    Obviously, the main items on the agenda were the AAVS1 requirements, but we also discussed several other matters. After defining the correct levels within the requirements, an overview was presented and we discussed the AAVS1 Product Breakdown Structure.

    The AAVS1 requirements have of course been derived from the LFAA functional and performance requirements contained in the SKA Phase 1 System Requirements Specification. However, since AAVS1 will be constructed with fewer antennas than the eventual LFAA array, the baseline and sensitivity are reduced. The AAVS1 main array will now be designed and built, and used in conjunction with MWA for verification. The AAVS1 requirements have been chosen such that the system can be used to validate the LFAA design. The scaling has been chosen such that the required functional performance of AAVS1 will be comparable with a single logical station.

    Our next major milestone is the AAVS1-DDR, 12th October 2015.


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  • 07/27/15--17:00: Summer student party
  • © Astron / JIVE

    The traditional Director's Summer Student party was held on July 15th. Due to building activities, it was a somewhat smaller affair than usual, just the students and their supervisors. Nevertheless it was a cheerful afternoon, despite bit of rain that caused us to invade Mike's house every now and then (making a lot of mess, too). The picture shows the students in their Astron hats, with Leonid representing JIVE, and with the genial host himself. From left to right:

  • Leonid Gurvits

  • Wen Zhigang (China) supervisor: Zsolt Paragi

  • Amidou Sorgho (ZA) supervisor: Erwin de Blok

  • David Bordenave (USA) supervisor: Michiel Brentjens

  • Marilyn Cruces (Chile) supervisor: Anne Archibald

  • Mike Garrett

  • Vikram Singh (USA) supervisor: Minnie Mao

  • Joseph Kania (USA) supervisor: Giuseppe Cimo

  • Sahba Yahya Hamid Ali (ZA) supervisor: Kelley Hess

  • Bahar Bidaran (India) supervisor: Betsey Adams

    For more information, see: http://astron.nl/astronomy-group/summer-school/projects/summer-student-projects . After the students left for bridge, a few hard-core die-hards stayed behind to get to the bottom of things.


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    © Aleksandar Shulevski

    On June 19 2015, version 17.0 of the LOFAR Imaging Cookbook has been released.

    The first version of the manual was published more than 4 years ago, when the first LOFAR commissioners met together during the commissioning Busy Weeks to test and validate the new reduction software for processing LOFAR data which was becoming available.

    Since 2010, the manual has developed significantly. Its content has become very comprehensive of all the data reduction steps needed to achieve outstanding LOFAR imaging results. The topics cover a range from data inspection to flagging, calibration, imaging, and source extraction. More recently, important chapters have been included on advanced methods to remove the A-team signal from the visibilities, advanced tools to inspect the calibration solutions, the automated self-calibration algorithm, and a description of the new LOFAR CEP 3 cluster.

    The LOFAR Imaging Cookbook is edited by Aleksandar Shulevski, who coordinates the large group of experienced LOFAR commissioners in charge of updating the chapters of the manual: G. van Diepen, T. J. Dijkema, G. Heald, F. de Gasperin, M. Iacobelli, J. McKean, M. Mevius, A. Offringa, E. Orru ́, D. Rafferty, C. Tasse, B. van der Tol, V. Vacca, N. Vilchez, R. van Weeren, W. Williams and S. Yatawatta.


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    © Rob Millenaar

    At ASTRON there is ample opportunity for lunch-time walks and runs in the Dwingelerveld National Park. There is even a "Sterrenwachtloop" (Observatory Run), connecting the Westerbork and Dwingeloo telescopes. On the last edition, your humble servant struggled through this picturesque 24 km run.

    Now that I find myself in South Africa the running continues, also at the site of the MeerKAT and (future) SKA1-Mid arrays in the Karoo. The other day I established two routes and lightly ran them to set a target for others to improve upon (which can be easily done). I have named these runs:

  • The Mad MeerKAT Dash (red and yellow). A 19 km jaunt from the Losberg facility to the core of MeerKAT, circling it and then retracing one's steps back to where one started.

  • The Losberg Loop (red and magenta). At just over 12 km in length this is a scenic stroll around the Losberg hill, over tracks that see more aardvarks than humans. Loose soil makes the going tough in places.

    I invite all ASTRON athletes to engineer an opportunity to come over to smash my times(*). Until then, I proudly hold the records.

    (*) But remember the admiring colonial homily: "Only mad dogs and Englishmen go out in the noon-day sun".


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    © Pictures taken by Niels Bos

    In July 1965, two papers appeared in ApJ Letters that changed our view of the Universe. The first was by Dicke, Peebles, Roll & Wilkinson from Princeton University, who postulated that there should be an excess temperature emission from the Universe. The existence and exact temperature of this cosmic microwave background would place important constraints on the Big Bang model for the Universe and its evolution to present day. Although they did not present a detection, their letter was motivated by another group, Penzias & Wilson at Bell Laboratories who, in the very next letter in the journal, reported the discovery of an excess temperature of 3.5 +/- 1 K from their antenna that could not be accounted for from either their telescope or the atmosphere. As it turned out, Penzias & Wilson had discovered the cosmic microwave background, winning them the Noble prize in Physics in 1978.

    50 years later, in July 2015, four young budding radio astronomers at the University of Groningen (RuG) have built their own radio telescope to make this measurement as part of their bachelors thesis projects. Bram Lap (right) designed and built the horn antenna, Maik Zandvliet (middle-right) constructed and tested the backend of the receiver system (amplifiers, filters and mount construction), Frits Sweijen (left) wrote the telescope control software and carried out test observations of the Sun, and Willeke Mulder (middle-left) devised the calibration scheme and made the final measurement of the CMB.

    Shown in the image is the team with their telescope, along with their supervisors, John McKean (middle; Staff Astronomer at ASTRON and Asst. Prof. at the RuG), Andrey Baryshev (left; Prof. at the RuG) and Ronald Hesper (right; Researcher at the RuG). The other pictures and data plot are from their first light observation on 2 July 2015, which detected the Sun (at 132 deg. and the buildings at the Kapteyn Astronomical Institute at > 155 deg.). The group have successfully detected an excess radio emission from the Universe with their telescope and are currently reducing the systematics in the receiver temperature calibration in order to make a precise measurement of the CMB temperature.

    The radio telescope will be used by future undergraduate classes to make measurements of the radio sky as part of the 3rd-year course "Introduction to Radio Astronomy", taught by John McKean at the RuG.

    The team would like to thank the staff at SRON and ASTRON for their help in building the telescope, and Prof. Jim Moran from Harvard University for his advice on carrying out the experiment.


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    © Emiel Brommer

    Half a year ago I've started my internship at ASTRON at the radiogroup. I got an assignment from Mark Ruiter and Erik van der Wal. My assignment was to build an automated low noise test system for measuring low noise amplifiers. These low noise amplifiers are widely used in radio telescopes, including Apertif, Lofar and SKA. This test system must be built with a spectrum analyzer.

    After learning the basics of noise theory, usage of the spectrum analyzer and getting used to Linux, which was completely new to me. I started working on the test system. I connected the spectrum analyzer to the network and communicated with it with Octave, which is the open source version of Matlab. I wrote some Octave functions to make the communication easier and with these functions I wrote the final script which handles to whole measurement starting with setting up the spectrum analyzer, then reading from the spectrum analyzer and calculating the noise figure and finally present it in a nice way to the user.

    On the right you can see one of my test measurements. The graph at the top shows the measured noise figure of an amplifier for two different supply voltages. At the bottom is a photo of the setup I used.

    During my internship I've learned a lot, too much to enumerate here. Some highlights are working with Octave, understanding and using network protocols, using a spectrum analyzer and noise theory.

    I had a very nice time at ASTRON and want to thank everyone for making that possible for me. The radiogroup received me well and I blended in easy. I would particularly like to thank Mark Ruiter and Erik van der Wal for their great support during my internship.

    Thanks to you all!


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    © Betsey Adams

    AGC 226067 is an intriguing source originally identified by its neutral hydrogen gas content (HI) in the ALFALFA HI survey with no apparent optical counterpart. Subsequent deep ground-based optical imaging revealed a blue fuzzy counterpart, identifying the system as a low-mass galaxy with an extremely high ratio of HI to stellar mass. Recently, a team led by Betsey Adams and including Tom Oosterloo used data from the VLA to show that the system consists of multiple components (both gas and stars) connected by low surface brightness HI. The image above shows the two main HI components in the white contours, and the red circles indicate the optical sources. Fully understanding the system will require better data but a likely explanation is that the main source (bottom of the image; gas and stars coincident with each other) is a dwarf galaxy undergoing some sort of interaction as it falls into the Virgo Cluster. Future surveys with Apertif will be able to detect many more systems like this one via their gas content.

    You can find more details in AGC 226067: A possible interacting low-mass system, Adams, E. A. K., et al. 2015, A&A in press, http://arxiv.org/abs/1507.01855


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    © NRAO

    Neutron stars and black holes are the two densest and most extreme forms of matter known in the Universe. When a neutron star or a black hole exists in a binary system with a companion star, they can accrete material and blast a small amount back out in powerful jets at relativistic speeds. Until recently, it was thought that black holes were the kings of jet formation - systems containing neutron stars were only visible when they were accreting vigorously, whereas black hole systems could be seen even when only nibbling on a trickle of material.

    This simple picture has now gotten a good deal more complex, with the publication of radio observations of PSR J1023+0038, one of the so-called "transitional millisecond pulsars", made with the Very Large Array and the European VLBI Network. The image shows an artist's conception of the PSR J1023+0028 system. The radio observations, which trace the strength of the jet, show that these particular neutron star systems make strong jets even though they are only accreting a small amount of material. It seems likely that a special combination of the neutron star spin period and magnetic field strength may be required to supercharge the system's jet, which also leads to strong production on gamma-rays. Further observations of other, newly discovered transitional millisecond pulsars are planned to further investigate what makes these systems punch above their weight in jet formation.

    The research was led by ASTRON scientist Adam Deller and included other ASTRON and JIVE scientists Javier Moldon, Jason Hessels, Anne Archibald, Zsolt Paragi, George Heald and Nicolas Vilchez. It was published this week in the Astrophysical Journal (ApJ, volume 809, issue 1: arXiv http://arxiv.org/abs/1412.5155 ).


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    © Stellenbosch University / ASTRON

    Since 2013 the MIDPREP staff exchange programme has strengthened collaborations between ASTRON and the South African Universities of Cape Town, Rhodes and Stellenbosch. As part of the MIDPREP staff exchange scheme fellowship, David Prinsloo from Stellenbosch University spent six weeks at ASTRON this summer working toward a sparse demonstrator array for the mid-frequency band of the Square Kilometre Array telescope.

    This work presents an antenna design for sparse arrays that utilises multiple orthogonal port excitation modes propagating in multi-conductor transmission lines in order to extend the field-of-view coverage of the array. The antenna illustrated in the figure - referred to as a quad-mode antenna - achieves hemispherical field-of-view coverage by integrating and co-locating two perpendicular bow-tie dipoles, four tapered slot antenna elements, and a conical monopole section into a single antenna excited through a quadraxial transmission line.

    Indicated alongside the quad-mode antenna, are the four orthogonal port excitation modes supported by the quadraxial transmission line - shown together with the respective far-field radiation pattern excited by each mode. Implemented in an array environment these four excitation modes can be used, collectively, to maximise the performance in terms of gain, sensitivity, or polarisation discrimination at each scan angle over a hemispherical field-of-view.

    In the present phase of the work the design of a tile of quad-mode antennas is underway. Once completed, this tile will serve as the base element in a sparse, irregular station proposed for the mid-frequency band of the Square Kilometre Array telescope.


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